feat(feature-flags): generalize driver-side cache

[cathleeny]

Summary

• Move and generalize the existing telemetry feature-flag cache into internal/featureflags; retain all returned values and add six typed getters.
• Share values by workspace ID, with normalized host as a fallback, and use authenticated transport supplied by each caller.
• Honor the connector-service TTL, falling back to 15 minutes when it is missing or invalid.
• Use the existing x/sync singleflight dependency to coalesce cold loads. During refresh, one caller fetches while other callers immediately read stale values; failed refreshes retain those values.
• Route telemetry through the Boolean getter and retain reference-counted cleanup.

No new cache library, kernel version bump, kernel-selection flag, or automatic pre-kernel GET is included.

Validation

Using Go 1.25:

• Full unit suite: go test ./... passed.
• Feature-flag and telemetry tests passed with the race detector.
• Cold-load/workspace and stale-reader concurrency tests passed for 30 repeated race-detector runs.
• The new stale-reader regression test failed before the fix and passed afterward.
• go vet ./... and golangci-lint passed (0 issues).

[cathleeny]

Replaced by #496, a direct upstream draft with identical commits. Closing this fork-based draft.