Fix dash-incompatible slash substitution and bump omos size threshold

v1.14.31b made it through smoke-base and validate-base (reclaim worked),
but two narrow bugs blocked the rest:

1. 'Derive platform slug' in the per-arch matrix jobs used bash
   ${PLATFORM_PAIR//\//-} which dash (/bin/sh in the runner) can't
   parse — 'Bad substitution'. Rewrote with 'tr / -'.

2. smoke-omos image size 3107 MB tripped the 3000 MB guardrail. All
   functional checks pass; the mempalace-toolkit bake-in from v1.14.30b
   added ~100 MB and the threshold was stale. Bumped to 3200 MB.

No image-level changes.

CHANGELOG.md

@@ -6,6 +6,15 @@ Tags follow `v{opencode_version}[letter]` — bare tag for the first build on a
 
 ---
 
+## v1.14.31c — 2026-05-01
+
+**CI: fix bash-specific parameter expansion and bump omos size threshold.**
+
+- **Fix:** `Derive platform slug` step in the per-arch matrix build jobs (`build-base`, `build-omos`) used `${PLATFORM_PAIR//\//-}` which is a bash parameter-expansion. The runner container executes step scripts via `/bin/sh` (dash), which errored with `Bad substitution`. Rewrote using `tr / -` which is POSIX and behaves identically. Both `build-base` and `build-omos` matrix jobs were blocked on this on `v1.14.31b`.
+- **Fix:** smoke-test image-size threshold for the `omos` variant bumped from 3000 MB to 3200 MB. The mempalace-toolkit bake-in added ~100 MB to omos; measured 3107 MB on `v1.14.31b`. All functional smoke checks (opencode, node, mempalace CLIs, toolkit wrappers, oh-my-opencode-slim) pass — this is a guardrail recalibration, not a performance concession. The underlying image genuinely grew.
+- The runner-disk reclaim step from v1.14.31b did its job: `smoke-base` and `validate-base` now pass cleanly. Only `smoke-omos` was blocked this iteration, and only on the threshold.
+- No image changes beyond what shipped in v1.14.31. Rebuild of v1.14.31 content only.
+
 ## v1.14.31b — 2026-05-01
 
 **CI: reclaim runner disk before `load: true` smoke builds.**