feat: add PostHog event tracking for CLI installs and upgrades (ENG-2277)

[devin-ai-integration]

Summary

Adds lightweight PostHog event tracking to the CLI with no new dependencies — uses only net/http and crypto/rand from stdlib. Mirrors the existing Sentry integration pattern (build-time key injection, opt-in consent via IsTrackingEnabled()).

Two events are tracked:

• "Installed CLI" — fires once per new CLI version on startup, deduplicated via ~/.blaxel/telemetry.json
• "Upgraded CLI" — fires after a successful bl upgrade with old_version and new_version properties

Events are fire-and-forget (async goroutine, 5s timeout). The PostHog API key is injected at build time via ldflags, same as sentryDSN.

Files changed:

• cli/core/posthog.go (new) — PostHog client, telemetry state persistence, UUID generation, deduplication logic
• main.go — adds posthogKey ldflag var, initializes PostHog when tracking is enabled
• cli/core/root.go — calls TrackCLIInstalled(version) in Execute()
• cli/upgrade.go — captures old version, detects new version post-upgrade, fires TrackCLIUpgraded
• .goreleaser.yaml / Makefile / .github/workflows/release.yaml — build-time key injection

Review & Testing Checklist for Human

• POSTHOG_KEY GitHub secret must be created before the next release. Verify that GoReleaser doesn't fail when POSTHOG_KEY env var is unset/empty (it may need to be set to "" like SENTRY_DSN).
• detectInstalledVersion() output parsing — the version command uses core.Print/core.PrintInfo which may emit ANSI color codes. Verify that parsing "Version: X.Y.Z" works correctly when the new binary's stdout is not a TTY (piped through exec.Command(...).Output()).
• FlushPosthog() uses time.Sleep(500ms) instead of a sync.WaitGroup — events could be silently dropped if the HTTP POST takes longer. Acceptable trade-off? Consider if a WaitGroup approach would be better.
• loadTelemetryState uses sync.Once — telemetry state is loaded once per process. This is fine for normal CLI usage but means the state won't be re-read within a single process lifetime (relevant for upgrade flow where TrackCLIUpgraded then updates state).
• Test the full flow locally: build with POSTHOG_KEY=phc_test make build-dev, run bl version twice, verify ~/.blaxel/telemetry.json is created with correct structure, and that only one "Installed CLI" event would fire.

Notes

• No unit tests added for posthog.go — the code is intentionally minimal and mirrors the untested Sentry client pattern in this repo.
• The getDistinctID() comment mentions "workspace email" but the implementation only uses a persisted anonymous UUID. The comment should be updated or the email lookup added if that's desired.
• The SDKs field in telemetryState is forward-looking — it's unused by the CLI but will be used by the SDK repos (sharing the same ~/.blaxel/telemetry.json file).

ENG-2277

Link to Devin session: https://app.devin.ai/sessions/43a6073c1fe54dbfb87b42e3dc01db56
Requested by: @Grotoma

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Note

Adds lightweight PostHog event tracking to the CLI using only stdlib (net/http, crypto/rand). Tracks "Installed CLI" once per new version (deduplicated via ~/.blaxel/telemetry.json) and "Upgraded CLI" after a successful bl upgrade. Mirrors the existing Sentry integration pattern with build-time key injection and opt-in via IsTrackingEnabled().

^{Written by Mendral for commit 9e1da84.}

[devin-ai-integration]

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[mendral-app]

LGTM

All previous comments are resolved. The final commit correctly adds FlushPosthog() to ExitWithError, ExitWithMessage, and Exit — since os.Exit skips deferred calls, these explicit flushes are necessary and correct. Ready to merge.

_{Tag @mendral-app with feedback or questions. View session}