[Xuepoo]

Summary

Adds the canonical user guide for the official Search plugin and a docs gate
that keeps the Configuration table in sync with the manifest.

Changes

• Add the required user sections (Overview, Install, Commands,
  Configuration, Keybindings, Troubleshooting) ahead of the developer
  material.
• Document the transcript read grant, opt-in capture, scoped query arguments,
  the two declared settings, and the typed E_HISTORY_* denials.
• State explicitly that copy fails closed in this release because the
  manifest declares no clipboard.write grant.
• Add tests/check-docs.mjs and just docs-check: fail on a missing or extra
  documented setting, on type/default drift, and on a wrong user-section
  order.

Verification

• just install + just check pass locally in the CI-equivalent environment
  (no Core checkout, so live-identity probes skip), including just docs-check.
• The live-identity test probes pin fresh origin/main... fails on untouched
  main in this workspace because 76fa42d6 is not an ancestor of the local
  bitty origin/main; it is unrelated to this change and skips on CI.
• Negative control: adding or removing a settings row fails the gate.

Priority: P2 | Area: plugin | Labels: docs, P2, area:plugin | Milestone: v0.1.0 | RFC: n/a | Task: n/a (user-facing plugin docs)