Merge #143: Add OutputSink abstraction for multi-destination output

1bd16d0 feat: make StandardSink public for composite usage (copilot-swe-agent[bot])
e8a98e5 fix: clippy doc-markdown warnings and format code (copilot-swe-agent[bot])
21f7884 feat: add OutputSink abstraction for multi-destination output (copilot-swe-agent[bot])
8bf1580 Initial plan (copilot-swe-agent[bot])

Pull request description:

  Implements composable sink pattern enabling output to multiple destinations (console, file, telemetry) simultaneously while maintaining backward compatibility.

  ## Changes

  **Core Abstractions**
  - `OutputSink` trait with `write_message(message, formatted)` method
  - `StandardSink` for stdout/stderr routing (default, backward compatible)
  - `CompositeSink` for fan-out to multiple sinks

  **Example Implementations**
  - `FileSink` for file output
  - `TelemetrySink` stub for observability integration

  **Integration**
  - `UserOutput` now uses `Box<dyn OutputSink>` instead of direct writers
  - Added `UserOutput::with_sink()` constructor
  - Made `StandardSink` public for composite usage
  - All existing constructors unchanged

  ## Usage

  ```rust
  // Default behavior unchanged
  let mut output = UserOutput::new(VerbosityLevel::Normal);

  // Console + File
  let composite = CompositeSink::new(vec![
      Box::new(StandardSink::default_console()),
      Box::new(FileSink::new("output.log")?),
  ]);
  let mut output = UserOutput::with_sink(VerbosityLevel::Normal, Box::new(composite));

  // Console + Telemetry
  let composite = CompositeSink::new(vec![
      Box::new(StandardSink::default_console()),
      Box::new(TelemetrySink::new("https://telemetry.example.com".to_string())),
  ]);
  let mut output = UserOutput::with_sink(VerbosityLevel::Normal, Box::new(composite));
  ```

  ## Notes

  - `flush()` is now no-op (documented limitation - StandardSink relies on OS line-buffering)
  - 15 new tests added for sink implementations and integration
  - External crates can implement custom sinks

  ## Related

  Part of #102 - User Output Architecture Improvements
  Depends on #127 - Message Trait (already merged)

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  <details>

  <summary>Original prompt</summary>

  >
  > ----
  >
  > *This section details on the original issue you should resolve*
  >
  > <issue_title>Proposal 9: Output Sink Abstraction</issue_title>
  > <issue_description>## Overview
  >
  > Introduce an `OutputSink` abstraction layer to enable writing output to multiple destinations beyond stdout/stderr. This proposal enables output to files, network endpoints, telemetry systems, or multiple destinations simultaneously through a composable sink pattern.
  >
  > ## Specification
  >
  > See detailed specification: [docs/issues/140-output-sink-abstraction.md](../docs/issues/140-output-sink-abstraction.md)
  >
  > ## Goals
  >
  > - [ ] Define `OutputSink` trait for output destinations
  > - [ ] Implement `StandardSink` for stdout/stderr (backward compatible)
  > - [ ] Implement `CompositeSink` for multiple destinations
  > - [ ] Enable extensibility for custom sinks (file, network, telemetry)
  > - [ ] Maintain backward compatibility with existing API
  > - [ ] Support testing with mock sinks
  >
  > ## Implementation Plan
  >
  > ### Phase 1: Core Trait and StandardSink (2 hours)
  > - [ ] Define `OutputSink` trait with `write_message()` method
  > - [ ] Implement `StandardSink` with stdout/stderr routing
  > - [ ] Add `StandardSink::default_console()` convenience constructor
  > - [ ] Ensure backward compatibility
  >
  > ### Phase 2: CompositeSink Implementation (1 hour)
  > - [ ] Implement `CompositeSink` for multi-destination output
  > - [ ] Add `new()` and `add_sink()` methods
  > - [ ] Test fan-out behavior to multiple sinks
  >
  > ### Phase 3: UserOutput Integration (1.5 hours)
  > - [ ] Update `UserOutput` to use `Box<dyn OutputSink>`
  > - [ ] Add `with_sink()` constructor for custom sinks
  > - [ ] Update `write()` method to use sink
  > - [ ] Verify backward compatibility
  >
  > ### Phase 4: Example Sinks and Documentation (1 hour)
  > - [ ] Implement `FileSink` as example
  > - [ ] Implement `TelemetrySink` as example (mock)
  > - [ ] Add comprehensive usage examples
  >
  > ### Phase 5: Testing (1.5 hours)
  > - [ ] Add unit tests for `StandardSink`
  > - [ ] Add unit tests for `CompositeSink`
  > - [ ] Add integration tests with `UserOutput`
  > - [ ] Create `MockSink` for test infrastructure
  >
  > ### Phase 6: Quality Assurance (1 hour)
  > - [ ] Run `./scripts/pre-commit.sh` and fix any issues
  > - [ ] Verify backward compatibility
  >
  > **Total Estimated Time**: 8 hours
  >
  > ## Acceptance Criteria
  >
  > ### Functional Requirements
  > - [ ] `OutputSink` trait defines contract for output destinations
  > - [ ] `StandardSink` maintains backward-compatible console output
  > - [ ] `CompositeSink` enables multi-destination output
  > - [ ] `UserOutput` works with any sink implementation
  > - [ ] Custom sinks can be implemented externally
  >
  > ### API Design Requirements
  > - [ ] Sink trait is simple and focused
  > - [ ] Composition is straightforward and intuitive
  > - [ ] Backward compatibility is maintained
  > - [ ] API follows Rust conventions
  >
  > ### Testing Requirements
  > - [ ] Unit tests cover sink implementations
  > - [ ] Integration tests verify `UserOutput` with different sinks
  > - [ ] Mock sink enables easy testing
  > - [ ] All existing tests continue to pass
  >
  > ### Quality Requirements
  > - [ ] Pre-commit checks pass: `./scripts/pre-commit.sh`
  > - [ ] Code follows project conventions
  > - [ ] Backward compatibility is verified
  >
  > ## Related
  >
  > - **Parent Epic**: #102 - User Output Architecture Improvements
  > - **Refactoring Plan**: [docs/refactors/plans/user-output-architecture-improvements.md](../docs/refactors/plans/user-output-architecture-improvements.md)
  > - **Depends On**: #127 - Use Message Trait for Extensibility
  >
  > ## Use Cases
  >
  > **Console Only (Default)**:
  > ```rust
  > let mut output = UserOutput::new(VerbosityLevel::Normal);
  > ```
  >
  > **Console + File**:
  > ```rust
  > let composite = CompositeSink::new(vec![
  >     Box::new(StandardSink::default_console()),
  >     Box::new(FileSink::new("output.log")?),
  > ]);
  > let mut output = UserOutput::with_sink(VerbosityLevel::Normal, Box::new(composite));
  > ```
  >
  > **Console + Telemetry**:
  > ```rust
  > let composite = CompositeSink::new(vec![
  >     Box::new(StandardSink::default_console()),
  >     Box::new(TelemetrySink::new("https://telemetry.example.com".to_string())),
  > ]);
  > let mut output = UserOutput::with_sink(VerbosityLevel::Normal, Box::new(composite));
  > ```
  >
  > ## Labels
  >
  > `enhancement`, `phase-2`, `user-output`, `P2`
  >
  > ## Priority
  >
  > P2 (Phase 2 - Polish & Extensions)
  >
  > ## Estimated Effort
  >
  > 8 hours
  > </issue_description>
  >
  > ## Comments on the Issue (you are @copilot in this section)
  >
  > <comments>
  > <comment_new><author>@josecelano</author><body>
  > **Parent Epic**: #102 - User Output Architecture Improvements
  >
  > This issue is part of Phase 2: Polish & Extensions of the User Output Architecture refactoring.</body></comment_new>
  > </comments>
  >

  </details>

  - Fixes #140

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ACKs for top commit:
  josecelano:
    ACK 1bd16d0

Tree-SHA512: de334e58ae78f41e2f2dc61204b82638cd9f082524250475089dbb3fbf6fbb4ffdbb35c5fa9686712ddec8dbc477361437fee316c611d60d8bd42f665faf3e0f

Author: josecelano