feat(a2a): implement M11 A2A types, discovery, and client (#97)

* feat(a2a): implement M11 A2A types, discovery, and client (#78)

New zeph-a2a crate with A2A protocol support:
- Protocol types (Task, Message, Part, Artifact, AgentCard) with camelCase serde
- JSON-RPC 2.0 envelopes and method constants
- AgentCardBuilder for runtime card generation
- AgentRegistry with well-known URI discovery and TTL cache
- A2aClient with send_message, stream_message (SSE), get_task, cancel_task
- Bearer token authentication for all client operations
- 42 unit tests covering serialization, discovery, and client operations

* feat(a2a): gate zeph-a2a behind optional "a2a" feature flag

Enabled by default. Build without A2A support via --no-default-features.

* chore: bump version to 0.6.0, update changelog and readme

Author: bug-ops

CHANGELOG.md

@@ -6,6 +6,21 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+## [0.6.0] - 2026-02-08
+
+### Added
+- New `zeph-a2a` crate: A2A protocol implementation for agent-to-agent communication (Issue #78)
+- A2A protocol types: `Task`, `TaskState`, `TaskStatus`, `Message`, `Part`, `Artifact`, `AgentCard`, `AgentSkill`, `AgentCapabilities` with full serde camelCase serialization (Issue #79)
+- JSON-RPC 2.0 envelope types (`JsonRpcRequest`, `JsonRpcResponse`, `JsonRpcError`) with method constants for A2A operations (Issue #79)
+- `AgentCardBuilder` for constructing A2A agent cards from runtime config and skills (Issue #79)
+- `AgentRegistry` with well-known URI discovery (`/.well-known/agent.json`), TTL-based caching, and manual registration (Issue #80)
+- `A2aClient` with `send_message`, `stream_message` (SSE), `get_task`, `cancel_task` via JSON-RPC 2.0 (Issue #81)
+- Bearer token authentication support for all A2A client operations (Issue #81)
+- SSE streaming via `eventsource-stream` with `TaskEvent` enum (`StatusUpdate`, `ArtifactUpdate`) (Issue #81)
+- `A2aError` enum with variants for HTTP, JSON, JSON-RPC, discovery, and stream errors (Issue #79)
+- Optional `a2a` feature flag (enabled by default) to gate A2A functionality
+- 42 new unit tests for protocol types, JSON-RPC envelopes, agent card builder, discovery registry, and client operations
+
 ## [0.5.0] - 2026-02-08
 
 ### Added

Cargo.lock

@@ -5,7 +5,7 @@ resolver = "3"
 [workspace.package]
 edition = "2024"
 rust-version = "1.88"
-version = "0.5.0"
+version = "0.6.0"
 authors = ["bug-ops"]
 license = "MIT"
 repository = "https://github.com/bug-ops/zeph"
@@ -35,12 +35,13 @@ toml = "0.9"
 tracing = "0.1"
 tracing-subscriber = "0.3"
 uuid = "1.20"
-zeph-channels = { path = "crates/zeph-channels", version = "0.5.0" }
-zeph-core = { path = "crates/zeph-core", version = "0.5.0" }
-zeph-llm = { path = "crates/zeph-llm", version = "0.5.0" }
-zeph-memory = { path = "crates/zeph-memory", version = "0.5.0" }
-zeph-skills = { path = "crates/zeph-skills", version = "0.5.0" }
-zeph-tools = { path = "crates/zeph-tools", version = "0.5.0" }
+zeph-a2a = { path = "crates/zeph-a2a", version = "0.6.0" }
+zeph-channels = { path = "crates/zeph-channels", version = "0.6.0" }
+zeph-core = { path = "crates/zeph-core", version = "0.6.0" }
+zeph-llm = { path = "crates/zeph-llm", version = "0.6.0" }
+zeph-memory = { path = "crates/zeph-memory", version = "0.6.0" }
+zeph-skills = { path = "crates/zeph-skills", version = "0.6.0" }
+zeph-tools = { path = "crates/zeph-tools", version = "0.6.0" }
 
 [workspace.lints.clippy]
 all = "warn"
@@ -54,11 +55,16 @@ authors.workspace = true
 license.workspace = true
 repository.workspace = true
 
+[features]
+default = ["a2a"]
+a2a = ["dep:zeph-a2a"]
+
 [dependencies]
 anyhow.workspace = true
 tokio = { workspace = true, features = ["full"] }
 tracing.workspace = true
 tracing-subscriber.workspace = true
+zeph-a2a = { workspace = true, optional = true }
 zeph-channels.workspace = true
 zeph-core.workspace = true
 zeph-llm.workspace = true

Cargo.toml

@@ -8,7 +8,7 @@
 [![MSRV](https://img.shields.io/badge/MSRV-1.88-blue)](https://www.rust-lang.org)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](LICENSE)
 
-Lightweight AI agent with hybrid inference (Ollama / Claude), skills-first architecture, semantic memory with Qdrant, and multi-channel I/O. **Cross-platform**: Linux, macOS, Windows (x86_64 + ARM64).
+Lightweight AI agent with hybrid inference (Ollama / Claude), skills-first architecture, semantic memory with Qdrant, A2A protocol support, and multi-channel I/O. **Cross-platform**: Linux, macOS, Windows (x86_64 + ARM64).
 
 ## Installation
 
@@ -45,7 +45,7 @@ docker pull ghcr.io/bug-ops/zeph:latest
 Or use a specific version:
 
 ```bash
-docker pull ghcr.io/bug-ops/zeph:v0.5.0
+docker pull ghcr.io/bug-ops/zeph:v0.6.0
 ```
 
 **Security:** Images are scanned with [Trivy](https://trivy.dev/) in CI/CD and use Oracle Linux 9 Slim base with **0 HIGH/CRITICAL CVEs**. Multi-platform: linux/amd64, linux/arm64.
@@ -261,7 +261,7 @@ context_budget_tokens = 8000  # Set to LLM context window size (0 = unlimited)
 
 ## Docker
 
-**Note:** Docker Compose automatically pulls the latest image from GitHub Container Registry. To use a specific version, set `ZEPH_IMAGE=ghcr.io/bug-ops/zeph:v0.5.0`.
+**Note:** Docker Compose automatically pulls the latest image from GitHub Container Registry. To use a specific version, set `ZEPH_IMAGE=ghcr.io/bug-ops/zeph:v0.6.0`.
 
 <details>
 <summary><b>🐳 Docker Deployment Options</b> (click to expand)</summary>
@@ -304,7 +304,7 @@ docker compose --profile gpu -f docker-compose.yml -f docker-compose.gpu.yml up
 
 ```bash
 # Use a specific release version
-ZEPH_IMAGE=ghcr.io/bug-ops/zeph:v0.5.0 docker compose up
+ZEPH_IMAGE=ghcr.io/bug-ops/zeph:v0.6.0 docker compose up
 
 # Always pull latest
 docker compose pull && docker compose up
@@ -408,6 +408,18 @@ Found a vulnerability? See [SECURITY.md](SECURITY.md) for responsible disclosure
 
 **Security contact:** Submit via GitHub Security Advisories (confidential)
 
+## Feature Flags
+
+| Feature | Default | Description |
+|---------|---------|-------------|
+| `a2a` | Enabled | [A2A protocol](https://github.com/a2aproject/A2A) client for agent-to-agent communication |
+
+Disable optional features for a smaller binary:
+
+```bash
+cargo build --release --no-default-features
+```
+
 ## Architecture
 
 <details>
@@ -420,7 +432,8 @@ zeph (binary)
 ├── zeph-skills     SKILL.md parser, registry, embedding matcher, hot-reload watcher
 ├── zeph-memory     SQLite + Qdrant, SemanticMemory orchestrator, summarization
 ├── zeph-channels   Telegram adapter (teloxide) with streaming
-└── zeph-tools      ToolExecutor trait, ShellExecutor with bash parser
+├── zeph-tools      ToolExecutor trait, ShellExecutor with bash parser
+└── zeph-a2a        A2A protocol client, agent discovery, JSON-RPC 2.0 (optional)
 ```
 
 </details>

README.md

@@ -0,0 +1,25 @@
+[package]
+name = "zeph-a2a"
+version.workspace = true
+edition.workspace = true
+authors.workspace = true
+license.workspace = true
+repository.workspace = true
+
+[dependencies]
+eventsource-stream.workspace = true
+futures-core.workspace = true
+reqwest = { workspace = true, features = ["json", "stream"] }
+serde = { workspace = true, features = ["derive"] }
+serde_json.workspace = true
+thiserror.workspace = true
+tokio = { workspace = true, features = ["sync"] }
+tokio-stream.workspace = true
+tracing.workspace = true
+uuid = { workspace = true, features = ["v4", "serde"] }
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }
+
+[lints]
+workspace = true