Fix the stale docs/ index and add a docs/plans/ pointer

IMPLEMENTATION.md's File Structure section listed six of the nine
docs/ files, missing TUI.md, TELEMETRY.md, and PERFTRACE.md.
Separately, docs/plans/ holds design-decision write-ups (including the
current authoritative source on live permission-system gaps) but
wasn't mentioned anywhere in AGENTS.md's Reference table, so an agent
following AGENTS.md's own "read /docs first" instruction had no way to
discover it.

Author: TheGreatAxios

AGENTS.md

@@ -90,3 +90,4 @@ Interchange is the standard library for this repo, consumed as published `@intx/
 - `docs/PLUGINS.md` — plugin manifest system and discovery
 - `docs/TELEMETRY.md` — what usage telemetry is collected and why
 - `docs/PERFTRACE.md` — local PerfTrace and opt-in OTEL export settings
+- `docs/plans/` — working notes and design decisions, not normative like the docs above; worth checking for known-issue write-ups before starting related work

docs/IMPLEMENTATION.md

@@ -137,7 +137,8 @@ src/
     command-catalog.ts, model-catalog.ts, chrome-state.ts, palette.ts,
     provider-setup.ts       Onboarding provider setup flow
 docs/
-  PRODUCT.md, ARCHITECTURE.md, IMPLEMENTATION.md, HOOKS.md, MCP.md, PLUGINS.md
+  PRODUCT.md, ARCHITECTURE.md, IMPLEMENTATION.md, TUI.md, HOOKS.md, MCP.md,
+  PLUGINS.md, TELEMETRY.md, PERFTRACE.md
 ```
 
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