fix(orb): require anonymization secret for export

[JSONbored]

Motivation

• Prevent accidental de-anonymization when ORB_ANONYMIZE is enabled but ORB_WEBHOOK_SECRET is missing or trivially short, which allowed empty-key HMACs to be shipped to the collector.

Description

• Require a stable per-instance anonymization secret when anonymization is on by returning early from exportOrbBatch if ORB_WEBHOOK_SECRET is not set or is shorter than 32 characters, and increment a metric gittensory_orb_export_errors_total with reason=missing_anonymization_secret (change in src/selfhost/orb-collector.ts).
• Update unit tests in test/unit/selfhost-orb-collector.test.ts to use a long test secret for normal flows and add regression tests that assert export fails closed and does not call the collector when the secret is missing or weak.
• Clarify the sample environment guidance in .env.example to document that ORB_WEBHOOK_SECRET is required (32+ characters) when ORB_ANONYMIZE=true.

Testing

• Ran the targeted unit tests with npm test -- --run test/unit/selfhost-orb-collector.test.ts, and all tests in that file passed.
• Verified git diff --check produced no spacing/conflict issues locally.
• Attempted to run the full gate with npm run test:ci, which began but was blocked by local type resolution for optional native dependencies (pg / ioredis) during tsc and thus did not complete in this environment.
• npm audit --audit-level=moderate was attempted but could not complete due to the registry audit endpoint returning 403 in this environment.

---

Codex Task

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (a6e32e7) to head (ac53908).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1267   +/-   ##
=======================================
  Coverage   95.03%   95.03%           
=======================================
  Files         177      177           
  Lines       19886    19889    +3     
  Branches     7166     7167    +1     
=======================================
+ Hits        18898    18901    +3     
  Misses        395      395           
  Partials      593      593           

Files with missing lines	Coverage Δ
src/selfhost/orb-collector.ts	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[JSONbored]

Superseded by #1257. #1257 keys Orb anonymization off a dedicated, domain-separated key derived from the GitHub App private key and makes exportOrbBatch a no-op when that key is absent (if (!appKey) return 0) — i.e. it already requires an anonymization secret for export, and does so with higher-entropy, single-purpose material than a reused webhook secret. Closing this narrower fix in favor of #1257 (rebased + green). Reopen if #1257 is dropped.