fix(core): reject writes with an oversized path segment

[khaliqgant]

Summary

• Local relayfile mounts materialize each path segment as a real filesystem entry. Most filesystems (ext4, APFS, ...) reject names over 255 bytes with ENAMETOOLONG — worse once a mount's own atomic-write suffix (.tmp-<random>) is appended to the leaf segment.
• A provider adapter that builds a path segment from unbounded content (a long title, etc.) could produce a canonical path no mount could ever materialize. Since nothing revisits an already-written path, that failure recurs forever on every sync cycle for that one file — no self-heal, no automatic recovery.
• Root cause of the incident this generalizes a fix for: relayfile-adapters#241 capped the reddit post title slug after a 275-byte segment made the reddit-monitor persona's sandbox mount fail continuously (mount sync cycle failed: ... file name too long, repeating indefinitely). That was a point fix in one provider adapter. This PR adds a shared guard at the two write entry points in @relayfile/core — writeFile (direct HTTP write API) and applyWebhookEnvelope (Nango/webhook sync path, the one that actually created the original bad file) — so no provider adapter can reintroduce this class of bug, ever.
• New MAX_PATH_SEGMENT_BYTES = 200 (comfortably under the 255-byte filesystem limit, leaving margin for mount-side suffixes) checked against each path segment's UTF-8 byte length, not JS string length.

Test plan

• packages/core/src/files.test.ts (new): unit tests for hasOversizedPathSegment, including UTF-8 byte-length correctness and a reproduction of the exact real oversized r/LocalLLaMA path that broke the mount. Plus writeFile integration tests confirming the guard rejects before touching storage and a normal path still proceeds.
• packages/core/src/webhooks.test.ts: new test confirming applyWebhookEnvelope rejects an oversized-path envelope with reason: "path_too_long" and never writes it to storage.
• Full packages/core suite: 42/42 pass (was 34 before this PR).
• npm run build in packages/core — clean.

Note

This does not retroactively fix any already-written oversized-path file (e.g. the specific stuck reddit-monitor orphan) — it only prevents new ones. That cleanup is tracked separately since it touches append-only production event history in relayfile-cloud, not this repo.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 86a86c89-0ed6-4829-ae09-6e2ab27241d0

📥 Commits

Reviewing files that changed from the base of the PR and between 10f4472 and 06b1708.

📒 Files selected for processing (4)

• packages/core/src/files.test.ts
• packages/core/src/files.ts
• packages/core/src/webhooks.test.ts
• packages/core/src/webhooks.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/max-path-segment-length

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[github-actions]

Relayfile Eval Review

Run: .relayfile/evals/runs/2026-07-15T18-01-05-950Z-HEAD-provider
Mode: provider
Git SHA: 85b915d

Passed: 4 | Needs human: 0 | Reviewable: 0 | Missing output: 0 | Failed: 0 | Skipped: 0

Human Review Cases

No reviewable human-review cases captured Relayfile output.

[cubic-dev-ai]

No issues found across 4 files

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