Add loop labels and bound MCP run output

[andrei-hasna]

Replaces the stale implementation in #3 with a bounded, current-main replacement.

Scope:

• add normalized loop labels across SQLite, PostgreSQL, API, CLI, SDK, OpenAPI, and MCP
• support current-label AND filtering for loops and runs
• add guarded MCP label mutation tools while preserving the existing mutation gate
• cap individual MCP run output at 32,000 characters and aggregate responses at 128,000 characters
• preserve existing workflow validation, shell rejection, annotations, and golden-schema coverage

Verification on exact head 344d6a49eaf4c8206175d62154605dccf75377e0:

• bun test: 936 pass, 28 skipped live-Postgres tests, 0 fail
• bun run typecheck
• bun run build
• bun run check:contracts
• bun run test:boundary
• staged and committed credential-pattern scans clean

Rebased onto main 3d4e38ed28167586dab4793c8c4b488327bd1ada. No deployment, publication, or database migration was applied.