chore(dev): add automated commit message generation workflow

Add jj-commit-message-generator agent and /sta:commit slash command to 
automate conventional commit message creation for STA project tasks.

Features:
- Agent analyzes jj diff and task specs to generate messages
- Slash command provides simple interface: /sta:commit T020
- Follows project conventions and TDD workflow patterns
- Uses lightweight Haiku model for fast generation

.claude/commands/sta-rustdoc.md

@@ -0,0 +1,41 @@
+# Add Missing Rust Documentation
+
+Run cargo clippy to identify items missing documentation, then add appropriate doc comments to all flagged items.
+
+## Instructions
+
+1. Run `cargo clippy --all-targets` to find missing documentation warnings
+2. Parse the output to identify all items (functions, structs, enums, modules, etc.) that need documentation
+3. For each missing doc comment:
+   - Read the relevant file to understand the context
+   - Add appropriate /// doc comments for public items
+   - Add appropriate //! module-level doc comments where needed
+   - Follow Rust documentation conventions:
+     - First line is a brief summary (imperative mood: "Creates", "Returns", not "Create", "Return")
+     - Add "# Errors" section for functions returning Result
+     - Add "# Panics" section if function can panic
+     - Add "# Examples" for complex public APIs
+     - Use #[must_use] attribute where appropriate
+4. After adding all documentation, run cargo clippy again to verify no missing docs warnings remain
+5. Report summary of what was documented
+
+## Documentation Style Guidelines
+
+- Modules (//!): Describe the module's purpose and main types/functions
+- Structs/Enums: Describe what the type represents
+- Functions/Methods: Describe what it does, parameters, return value
+- Fields: Document public fields with ///
+- Keep it concise but informative
+- Use markdown formatting for code examples
+
+## Example Output Format
+
+After completion, report:
+
+Added documentation to:
+- Module: src/domain/relay/entity.rs (module-level docs)
+- Struct: Relay (3 public methods documented)
+- Function: toggle() at src/domain/relay/entity.rs:XX
+
+Total: X items documented
+Clippy warnings resolved: X