flow: store hygiene flow/gc + flow/forget + 9 tests

flow/gc drops terminal (done/cancelled) records, keeps live suspended flows, returns
count removed; flow/forget id drops one terminal record and refuses live flows.
Bounds unbounded store growth (retention/GC). Bumped conformance sx_server timeout
to 540s for the 10-suite run under CPU contention. 151/151 across 10 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/flow-on-sx.md

@@ -16,7 +16,7 @@ federation extension via fed-sx for remote-node execution.
 
 ## Status (rolling)
 
-`bash lib/flow/conformance.sh` → **142/142** (Phases 1-7 complete)
+`bash lib/flow/conformance.sh` → **151/151** (Phases 1-7 complete; +store hygiene)
 
 ## Ground rules
 
@@ -141,6 +141,9 @@ something operators and authors actually use. Accumulation, not a rewrite.
 - [x] introspection API — `flow/status id`, `flow/result id`, `flow/list`,
   `flow/pending` (operator view of what each suspended flow awaits). 12 tests in
   `tests/api.sx`.
+- [x] store hygiene — `flow/gc` drops terminal (done/cancelled) records keeping
+  live suspended flows (returns count); `flow/forget id` drops one terminal record
+  and refuses live flows. Bounds unbounded store growth. 9 tests in `tests/hygiene.sx`.
 - [x] `tap` — side-effecting pass-through node (logging/metrics) that returns input
 - [x] `recover` — complement to try-catch for the fail-VALUE channel: run node; if it
   yields `(fail ...)`, run a recovery node on the reason