feat: add install_completed + first_rule_injected telemetry with hash-based cohort assignment

[Gradata]

Adds lab experiment telemetry to unblock GRA-221 (install_time_to_first_rule measurement).

Changes

• New hooks/lib/telemetry.js: generateInstallId, getCohort (hash-based 50/50 split), emitInstallCompleted, emitFirstRuleInjected
• setup/install.js: emits install_completed on install with deterministic cohort
• hooks/session-stop.js: emits first_rule_injected once per install when rules are active
• Install ID persisted to ~/.gradata/install_id for cross-hook read
• Guard file prevents duplicate first_rule_injected emissions

Verification

• All three files pass node --check
• Cohort assignment: hash(installId)[0] < '8' → control, else treatment
• Telemetry calls are best-effort (non-blocking, never throw)

Unblocks: GRA-221 (measurement window can open once this ships)

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: dee2bab0-0b70-4b66-8ee7-463778646579

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/telemetry-install-cohort

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}