fix(coding-agent): separate overflow-retry fit check from auto-continue band

Addresses PR #3412 review (roboomp blocking + codex P2 + Copilot nits):

- The overflow/incomplete retry no longer reuses the COMPACTION_RECOVERY_BAND
  hysteresis. Reusing it required residual context below 0.8×threshold, which
  turned a recoverable overflow (e.g. 150k on a 200k window, under the ~170k
  threshold) into a manual dead-end. Add #compactionCreatedRetryFit, which
  measures the rebuilt prompt against the usable fit budget
  (contextWindow - effectiveReserveTokens) and is evaluated AFTER the failed
  assistant is dropped, so the just-failed turn is excluded.
- The threshold auto-continue keeps the stricter recovery-band check
  (#compactionCreatedHeadroom) — that path is the snapcompact thrash guard.
- Split the no-progress warning so each path warns on its own signal.
- Alias errorIsFromBeforeCompaction as assistantPredatesCompaction in the
  threshold-usage path for clarity; de-duplicate the rationale comment.

Adds two regression tests (recoverable overflow retries; non-fitting overflow
pauses + warns), mutation-verified against the band-vs-fit split.

(cherry picked from commit f8cf45b2740f04d8776a84521ba563b779d65b25)

packages/coding-agent/CHANGELOG.md

@@ -118,6 +118,7 @@
 - Fixed `omp install` of legacy pi extensions failing with `Cannot find module '/$bunfs/root/packages/coding-agent/src/extensibility/typebox.js'` on every released `omp-<platform>-<arch>` binary. Commit `dc5c93462f` removed worker entrypoints from `scripts/ci-release-build-binaries.ts`; the inline comment then claimed the legacy-shim and package-barrel entrypoints (`typebox.ts`, `legacy-pi-{ai,coding-agent}-shim.ts`, `packages/{agent,natives,tui,utils}/...`) were "still" passed to `bun build --compile`, but they had never been re-added. The release binaries shipped without those files in bunfs, so `legacy-pi-compat.ts` redirected `typebox` imports to a bunfs path that didn't exist. `__resolveTypeBoxShimPath` now mirrors `__validateLegacyPiPackageRootOverrides` (#2168) by dropping the override when the shim file is missing, so a missing shim falls through to native `node_modules` resolution instead of emitting a dead bunfs URL ([#3414](https://github.com/can1357/oh-my-pi/issues/3414)).
 - Fixed every legacy `@(scope)/pi-*` and `@sinclair/typebox` import failing to load on the `omp-darwin-arm64` release binary (and any other `omp` built with Bun 1.3.14). `__validateLegacyPiPackageRootOverrides` and the `rewriteLegacyPiImports` emit path both depended on `--compile` extras being reachable as `/$bunfs/root/...` filesystem entries, but Bun 1.3.14 stopped exposing them through every API (`fs.existsSync`, `Bun.file().exists()`, `Bun.resolveSync`, `await import()` on the bunfs path or its `file://` URL all fail; only `/$bunfs/root/<binary-name>` itself answers). `legacy-pi-compat.ts` now keeps a JS-heap reference to every bundled pi-* surface in a lazy-loaded sibling `legacy-pi-bundled-registry.ts` and serves them through an `omp-legacy-pi-bundled:` virtual namespace whose `Bun.plugin().onLoad` returns synthetic re-exports — no bunfs path ever leaves the module in compiled mode, and dev / source-link / installed-package modes keep the historical `file://` rewrite. The matching `--compile` extras in `scripts/build-binary.ts`, the shared `scripts/binary-entrypoints.ts` list, and the dead `BUNFS_PACKAGE_ROOT` / `bunfsPath` / `__computeBunfsPackageRoot` / `__joinBunfsPath` helpers are gone. ([#3423](https://github.com/can1357/oh-my-pi/issues/3423))
 - Fixed auto-compaction thrashing on a session whose single most-recent kept turn already exceeds the compaction threshold. `prepareCompaction` keeps that turn verbatim (`findCutPoint` never cuts at tool results), so the rewritten context stays above threshold; the context-full / snapcompact success tail scheduled the agent-authored auto-continue (and the overflow/incomplete retry) unconditionally, so the next `agent_end` re-entered `#checkCompaction` over the same oversized tail and re-fired forever. This is the residual loop left after #3247 capped snapcompact's own frame projection — once the frame cap drops below one frame, snapcompact is skipped and the context-full summarizer path still made no headroom. `#runAutoCompaction` now gates the continuation/retry on a post-maintenance headroom check (`#compactionCreatedHeadroom`, sharing shake's `COMPACTION_RECOVERY_BAND` hysteresis from #2275); when a pass frees too little it pauses automatic maintenance and emits a single warning instead of looping. The post-turn threshold check also ignores an assistant's stale pre-compaction `usage` so the scheduled auto-continue cannot re-trip on the kept assistant's old high token count.
+- Fixed auto-compaction thrashing on a session whose single most-recent kept turn already exceeds the compaction threshold. `prepareCompaction` keeps that turn verbatim (`findCutPoint` never cuts at tool results), so the rewritten context stays above threshold; the context-full / snapcompact success tail scheduled the agent-authored auto-continue (and the overflow/incomplete retry) unconditionally, so the next `agent_end` re-entered `#checkCompaction` over the same oversized tail and re-fired forever. This is the residual loop left after #3247 capped snapcompact's own frame projection — once the frame cap drops below one frame, snapcompact is skipped and the context-full summarizer path still made no headroom. `#runAutoCompaction` now gates the threshold auto-continue on a post-maintenance headroom check (`#compactionCreatedHeadroom`, sharing shake's `COMPACTION_RECOVERY_BAND` hysteresis from #2275) and the overflow/incomplete retry on a separate fit check (`#compactionCreatedRetryFit`, measured after the failed turn is dropped) so a recoverable overflow that fits the window still retries; when a pass frees too little for the relevant path it pauses automatic maintenance and emits a single warning instead of looping. The post-turn threshold check also ignores an assistant's stale pre-compaction `usage` so the scheduled auto-continue cannot re-trip on the kept assistant's old high token count.
 
 ## [16.1.17] - 2026-06-24