refactor: shared contract package, consistent API errors, and release hardening

[caiopizzol]

Consolidates the codebase around a single API contract and closes the gaps that made the four surfaces (API, MCP, CLI, web) drift apart. The driver: types, client code, and validators were hand-copied across packages and had already diverged, and the two published packages were never typechecked in CI.

• New internal packages/contract holds the canonical types, HTTP client, and validators; MCP, CLI, and web consume it. Published packages bundle its source at prepack, so npm consumers never see a workspace dependency.
• API errors are now a uniform JSON envelope, deploy orchestration moved out of the route layer into a tested service, and the blind try/catch SQLite migrations became an ordered user_version runner that fails loudly.
• The 3,100-line MCP entrypoint is split into per-domain tool modules with first test coverage, and the README tool table is generated from source (CI fails if stale).
• GitHub URL validation is strict (exact github.com/www.github.com, https only) and identical across API and clients; the old check accepted lookalike hosts.
• Release semantics: semantic-release-monorepo filters commits to each package directory, so contract-only commits would have released nothing while silently changing bundled output. A local shareable config widens the filter to include packages/contract. Trade-off: it imports semantic-release-monorepo internals, so bumping that dependency needs a release-path test first.
• docs/architecture.md records the surface hierarchy and the decision that self-update, drain, backup, and cleanup are core agent-ops features, not scope creep.