feat: stamp product: failproofai-oss on all PostHog events

[NiveditJain]

What

Adds a product: "failproofai-oss" property to every PostHog event, stamped at all four telemetry choke points so no event can be missed:

Channel	File / fn	Covers
Hooks / audit	src/hooks/hook-telemetry.ts (trackHookEvent)	all hook events + failproofai audit events
Server	lib/telemetry.ts (trackEvent)	app_started, etc.
Web UI	lib/client-telemetry.ts (captureClientEvent)	pageviews, all usePostHog().capture(...), client errors
npm lifecycle	scripts/install-telemetry.mjs (trackInstallEvent)	install / uninstall events

How

• New single source of truth POSTHOG_PRODUCT = "failproofai-oss" in src/posthog-key.ts, reused by the three TypeScript channels.
• The standalone scripts/install-telemetry.mjs inlines the same literal (with a comment) — it runs as a raw npm-lifecycle script and can't import the TS module at install time, mirroring how it already inlines its own API key.
• Honors FAILPROOFAI_TELEMETRY_DISABLED=1 like all other telemetry (no behavior change there).

Tests

• Updated the two existing real-payload tests (telemetry.test.ts, client-telemetry.test.ts) to assert product.
• Added two new test files (__tests__/hooks/hook-telemetry.test.ts, __tests__/scripts/install-telemetry.test.ts) covering the hook + install channels that previously had no payload assertions.
• All four channels now assert product: "failproofai-oss" on the actual emitted body.

Verified locally: lint (0 errors), tsc --noEmit clean, 1690 unit tests pass, 298 e2e tests pass.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 0dcf11bb-97a1-430e-a862-991e5dbd0dfb

📥 Commits

Reviewing files that changed from the base of the PR and between 2911b74 and 2e43f97.

📒 Files selected for processing (10)

• CHANGELOG.md
• __tests__/hooks/hook-telemetry.test.ts
• __tests__/lib/client-telemetry.test.ts
• __tests__/lib/telemetry.test.ts
• __tests__/scripts/install-telemetry.test.ts
• lib/client-telemetry.ts
• lib/telemetry.ts
• scripts/install-telemetry.mjs
• src/hooks/hook-telemetry.ts
• src/posthog-key.ts

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