fix: zero cargo warnings, zero actionlint warnings

[MethodWhite]

• Fix actionlint: pass github expressions via env vars instead of inline
• Fix 27 cargo warnings: unused imports, dead code, privacy, mut
• cargo fix auto-fixed 9 warnings
• Made types pub where needed (ColdFragment, AssistantDecision, etc.)
• Added #[allow(dead_code)] on struct fields intentionally unused

Description

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change
• Database / schema
• MCP tool
• Refactor (no functional changes)
• Documentation

Checklist

• cargo build --release succeeds
• cargo test passes
• cargo clippy has no new warnings
• cargo fmt has been run
• No eprintln! noise under SYNAPSIS_QUIET=1
• Errors use JSON-RPC error field, not result.content
• New tools have inputSchema with types, defaults, descriptions
• Changes are backward-compatible (or marked as breaking)
• Database migrations have version bump in schema_version

Database changes

• Schema migration required (version: ___)
• FTS index needs rebuild
• No database changes

Related issues

Closes #...