feat(mcp): expose the drive docs surface as MCP tools

[newhoggy]

Summary

ADR-0076 §12 defers MCP tools for the Docs surface,
following the CLI-first deferral ADR-0070 §9, ADR-0071 §11 and ADR-0073 §12
already set. This is the Docs counterpart of #1614 (Sheets), and should
probably be picked up alongside or after it — they share every design
question and the answers should match.

The CLI surface shipped in #1628 and #1639: drive docs info/read (ungated)
and drive docs replace/append/create (gated on docs-write).

What should be easy

The read tools mirror drive_sheets_info/drive_sheets_read almost exactly.
src/mcp/drive_tools.rs already has the shape, and the engines return typed
outcomes that serialize cleanly.

What needs a decision

Should the mutating verbs be exposed at all? #1614 faces the same question
for Sheets, and the answer should be the same for both — it is a decision
about the MCP surface, not about Docs.

Two properties make it safer here than it might look, both worth stating in
whatever decision gets made:

• The gate cannot be bypassed by an MCP caller. FilesApi's mutating
  methods and DocsApi::batch_update are pub(in crate::drive), so a tool
  handler physically cannot call them without going through a gated engine.
  That is a compile-time property, not a convention.
• Logging cannot be bypassed either. record_attempt lives inside the
  engine rather than the CLI layer, specifically so a future MCP caller still
  produces a drivemutation record — including for a Blocked outcome, which
  makes zero API calls.

And one that makes it harder:

• --dry-run has no natural MCP analogue. A tool call is not a shell
  invocation the user inspects before running. Either the preview becomes a
  separate tool, or the mutating tools are exposed without one — which
  changes the safety story ADR-0076 §7 relies on.

Also worth settling

• Whether is_stale_revision's outcome needs a distinct machine-readable
  shape, so an agent can tell "re-run this" from "this was wrong".
• Whether the occurrence-count estimate should be surfaced at all, given
  ADR-0076 §7 makes it display-only and explicitly non-authoritative — an
  agent is more likely than a human to treat a number as a fact.

Refs #1615, #1628, #1639, #1614, ADR-0076 §12