chore: drop internal planning doc from PR (not a repo convention)

docs/plans/ isn't an existing convention in this project -- it was carried
over from unrelated personal tooling and doesn't belong in an upstream
contribution. Removes it from the branch; the code, tests, and
user-facing docs changes are unaffected. Matches the closed PR's own
precedent (commit 8559dba on feat/per-session-flag-isolation).

Author: eggrollofchaos