fix(platform-node): harden file storage

[LukasParke]

What

• make file-storage key encoding bijective for underscores and __
• move storage reads/writes to asynchronous filesystem operations with unique temp files and atomic rename
• preserve legacy on-disk keys through guarded read migration
• keep list() on an instance-local key index and batch getMany() reads

Why

The previous filename encoding decoded literal __ as percent escapes, which could make persisted keys disappear from enumeration. Synchronous checkpoint I/O also blocked the event loop. Changing the encoder alone would orphan files written by existing releases, so reads retain a collision-safe legacy fallback and migrate legacy files without overwriting a concurrent canonical write.

This semantically ports the file-storage portion of fork/port/openrouter-fixes commits b9905ac5 and 653b3986 onto current main after #67 and #68. It is not a cherry-pick.

createFileStorage remains single-writer-per-root for list() consistency; this pre-existing deployment assumption is now explicit in its JSDoc.

Test plan

• cd packages/platform-node && bun test — 74 pass, 1 benchmark skipped
• cd packages/platform-node && bun run typecheck
• bun test — 2,058 pass, 6 integration/benchmark skips
• bun run lint
• sentrux check .
• sentrux gate .
• separate clean-context Pi review and re-review; confirmed races fixed, no remaining findings

Breaking behavior

None intended. Existing legacy files remain readable. Multiple live adapters pointing at the same root are not supported for list() coherence.

[vercel]

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[LukasParke]

Agent: The compat check failed before exercising this change because the workflow received an empty OPENROUTER_API_KEY (OPENROUTER_API_KEY is not set). CI, DCO, and the structural gate passed. This requires the upstream repository secret/workflow configuration or a maintainer rerun with secrets available; there is no code failure to remediate in this PR.