[connectors] Phase 1c: convert web-search + github/meta to manifests (proof, non-breaking)

[serge-ivo]

Convert web-search (simplest: api-key, one GET tool) first as the proof, then github/meta — keeping their one custom branch as an optional per-tool handler escape hatch. Tool names stay identical (persisted capabilities.tools / consent keys untouched); the existing dedicated tests (100%) are the behavior contract. Part of #143.

[serge-ivo]

Prerequisite (design decision): the existing connectors have custom OUTPUT contracts that differ from the executor's {status,data} (e.g. web-search returns {query,count,results}; meta branches text-vs-template). Per docs/connector-manifest.md the chosen path is the optional per-tool handler escape hatch — a manifest tool is "request-as-data OR a named code handler". Implement that first, then convert web-search (proof) → github → meta, keeping each one's dedicated 100% test suite as the behavior contract.

[serge-ivo]

Escape hatch + web-search done (commit f618e79):

• Manifest tools can now be request-as-data OR a named code handler. Built-in manifests only — sanitizeConnectorManifest never emits handler, so untrusted/creator manifests stay data-only (can't reference arbitrary code). compileConnector(m, handlers) binds the named fn and throws at compile time on an unknown name.
• web-search is now defined as a declarative manifest (WEB_SEARCH_MANIFEST) — shape as data, its one tool keeps its custom {query,count,results} envelope via the handler escape hatch. Registered via compileConnector, behavior-identical (all 16 web-search + 171 connector tests green; the tool resolves from the registry unchanged).

Remaining on this issue: convert github (4 tools, app auth, ownerOf validation + per_page clamp + issue delegation + create_issue) and meta (2 tools, text-vs-template branching) to the same pattern — data shape + a handler per custom tool. Their existing 100% dedicated suites are the behavior contract.