refactor: one durable pending-action store — the MCP park migrates up, the bespoke mechanism retires (ADR-0047 §5.4 follow-up)

[alicoding]

Summary

ADR-0047 §5.4's homed follow-up: guardrailsvc.PendingActionStore is
the generic pending-action model brought up to MCPWriteRecord's own
guarantees (restart survival, retention-window sweep, at-most-once
apply-on-approve, an opaque Payload for restart-safe re-dispatch).

• guardrailsvc.RequestGuardedAction now parks through the shared
  store instead of an in-memory-only map.
• mcpsvc's gated-write lifecycle (gateWrite/ResolveMCPWrite/
  CancelMCPWrite/PendingMCPWrites/ResolvedMCPWrites/
  check_write_status) migrates onto the same store.
• MCPWriteRecord, its bespoke settings key, loadWrites, and
  persistWritesLocked are deleted.
• A settings file carrying the old mcp-pending-writes key upgrades
  automatically (mcpsvc.MigrateLegacyPendingWrites, wired in
  main.go ahead of guardrailsvc.NewGuardrailService's own
  construction, so the shared store never loads a stale
  pre-migration view) — a user's parked write survives the upgrade.
• Retention is now a per-caller parameter on the shared store
  (mcpsvc's own mcpWriteExpiry var), not a single global, so two
  different consumers of the store can each state their own window.

Test plan

• Every existing MCP-write/guardrail Go test passes UNMODIFIED
  (including the two persist-failure tests, which turned out not
  to need migration — the constructor-time default store is real
  and backed by the same settings.Store already passed in).
• New guardrailsvc tests pin the same persist-failure property
  directly against the shared store (kept alongside, not instead
  of, the still-passing mcpsvc-level ones).
• New restart-survival, at-most-once, courtesy-window, and expiry
  tests for PendingActionStore itself, -race clean.
• New upgrade-path test: seed the old key/shape, migrate, confirm
  the record lists, resolves, and re-dispatches correctly, and the
  old key clears.
• Full local gates: go vet, golangci-lint, gocognit-new,
  depguard-new, comment-hygiene, file-loc-limit, go test ./... -race (one pre-existing, already-quarantined, unrelated
  clipboard flake — confirmed also present on unmodified
  origin/main on this machine).
• Relevant e2e specs green: guardrail.spec.ts,
  guardrail-authoring.spec.ts, guardrail-review.spec.ts,
  mcp-write-approval.spec.ts, mcp-write-cancel.spec.ts,
  mcp-write-staleness.spec.ts, atlas-mcp-write-approval.spec.ts,
  approval-prompt.spec.ts (20/20).

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https://claude.ai/code/session_012im1JxQQV2ahnXzZDdVmZq