docs: add issue for secret type introduction

- Create simple issue document with overview and goals
- Link to comprehensive refactor plan and ADR
- Include acceptance criteria for tracking progress

Author: josecelano

docs/issues/introduce-secret-type-for-sensitive-data.md

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+# Introduce Secret Type for Sensitive Data
+
+**Issue**: #TBD
+**Related**: [ADR: Secrecy Crate for Sensitive Data](../decisions/secrecy-crate-for-sensitive-data.md)
+
+## Overview
+
+Replace all primitive `String` types used for sensitive data (API tokens, passwords, database credentials) with the industry-standard `secrecy` crate's `Secret<String>` type. This enhances security by preventing accidental exposure through logging/debug output and enabling automatic memory zeroing.
+
+## Refactor Plan
+
+See detailed refactor plan: [docs/refactors/plans/secret-type-introduction.md](../refactors/plans/secret-type-introduction.md)
+
+The plan includes:
+
+- 10 proposals across 4 phases
+- Comprehensive inventory of 16 secret fields across all DDD layers
+- Progress tracking with detailed implementation checklists
+- Timeline and testing strategy
+
+## Goals
+
+- [ ] Replace 16 string-based secret fields with `Secret<String>` type
+- [ ] Prevent accidental secret exposure in logs and debug output
+- [ ] Enable secure memory zeroing for sensitive data
+- [ ] Update documentation with secret handling guidelines
+
+## Acceptance Criteria
+
+> **Note for Contributors**: These criteria define what the PR reviewer will check. Use this as your pre-review checklist before submitting the PR to minimize back-and-forth iterations.
+
+**Quality Checks**:
+
+- [ ] Pre-commit checks pass: `./scripts/pre-commit.sh`
+
+**Task-Specific Criteria**:
+
+- [ ] All 16 secret fields converted to `Secret<String>` (tracked in refactor plan)
+- [ ] No secrets appear in debug/display output
+- [ ] All unit tests pass with updated secret types
+- [ ] All E2E tests pass with secret handling
+- [ ] AGENTS.md updated with secret handling rule
+- [ ] Documentation includes examples of proper secret usage