refactor: consolidate truncate.ts and truncation.ts cleanup logic

[sahrizvi]

Deferred from the v0.9.6 release review (Tech Lead persona, P2).

Context

The v0.9.6 truncation-cleanup bug (#1112, fixed in #1113) had to be patched in two near-identical modules:

• packages/opencode/src/tool/truncate.ts — Effect service (modern path)
• packages/opencode/src/tool/truncation.ts — legacy module (used by bootstrap, bash, prompt)

Both compute the same mtime-based aging cutoff, both handle stat-failure the same way (fail-safe: keep the file), both were subject to the same 48-bit-ID timestamp-wrap bug because they were both computing cutoffs from decoded Identifier timestamps.

What to do

• Extract the mtime-aging cleanup into a shared helper (e.g. packages/opencode/src/tool/cleanup-common.ts or similar), used by both modules
• Remove the duplicated stat/mtime/fail-safe logic from truncate.ts and truncation.ts
• Ensure the regression test in packages/opencode/test/tool/truncation.test.ts exercises the shared helper (both direct and via each module's public API)

Pre-existing duplication, not introduced by #1113 — but the #1113 fix touched both files without consolidating them. Consolidating now costs one refactor; leaving it means the next truncation-cleanup change has to be applied twice again.

Priority

P2 — no user impact, but a footgun for the next contributor working in this area.