Codify plan-save ordering and add plan execution summaries

The plan for this build was written to docs/plans/ only after Step 0's
implementation had already started, instead of as its own first action
right after ExitPlanMode. CLAUDE.md said *where* to save plans but not
*when* relative to other work, so that ordering wasn't actually enforced.
Makes it explicit: copying the plan into docs/plans/ and committing it is
its own checkpoint that blocks starting Step 0.

Also adds a docs/plans-executions/ convention: one running, chronological
summary file per plan, appended to after each completed step, for a
human-readable narrative of how the plan actually went (judgment calls,
spec gaps, surprises) without duplicating the plan or the diff.

Backfills the Step 0 entry for the proxy-operator plan retroactively.

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -86,6 +86,30 @@ Include the same timestamp in the plan's header, e.g.:
 Create `docs/plans/` on first use. Plan files are committed to the repo so other
 contributors can review historical decisions.
 
+**Ordering matters:** plan mode itself restricts writes to the default
+`~/.claude/plans/` location — that's a hard tool restriction during plan mode, not a
+choice. So the copy into `docs/plans/` cannot happen until *after* `ExitPlanMode`.
+When it happens, it must be the very first action taken post-exit, committed on its
+own, *before* any implementation work starts (branching, scaffolding, editing code).
+Do not fold the plan-file commit into the first implementation commit, and do not
+defer it until "later" in the same turn — treat "save and commit the plan" as its own
+checkpoint that blocks starting Step 0.
+
+### Plan execution summaries
+
+After finishing each step of an approved plan, append a short, human-readable summary
+to `docs/plans-executions/YYYY-MM-DD-HHMM-<slug>.md` — same timestamp and slug as the
+originating plan in `docs/plans/`, so the two files pair up 1:1. Create the file (and
+`docs/plans-executions/` on first use) after the first step completes; append a new
+section per step after that, in chronological order (bottom of the file), not
+newest-first like `CHANGELOG.md`.
+
+Keep each entry light — a few sentences on what was done, plus anything genuinely
+interesting or non-obvious about how it went (a judgment call made, a spec gap found,
+something that didn't work as expected). This is not a duplicate of the plan or the
+diff; skip steps that went exactly as planned with nothing worth flagging. Commit the
+summary update together with that step's implementation commit.
+
 ### Changelog
 
 Maintain a running changelog in `CHANGELOG.md` at the repo root. After every