Keep working notes out of the repository

docs/plans/ collected fifteen analysis documents in a day. They are
scaffolding for decisions, and a plan describing finished work is a stale
doc waiting to happen — the same failure that had PRODUCT.md documenting
behaviour the code never had.

Ignored rather than curated: durable conclusions belong in the reference
docs or the tracking issue. The two committed files are removed from git
and left on disk.

Author: TheGreatAxios

.gitignore

@@ -24,3 +24,6 @@ evals/capability/results/*
 !evals/capability/results/baseline-0286.json
 local-harness-bench/
 subagents/
+
+# Working notes and design spikes — local only, never committed
+docs/plans/

AGENTS.md

@@ -90,4 +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/` — non-normative working notes and design spikes; check before starting related work
+`docs/plans/` is git-ignored: local working notes and design spikes only. Durable conclusions belong in the docs above or in the tracking issue — never left as a plan file, which is a stale doc waiting to happen.