[nightly-telemetry] Add dictation start attempt bucket

[r3dbars]

Summary

• add start_attempt_bucket to dictation_start_failed analytics for microphone-start timeouts
• keep raw start_attempts out of PostHog while preserving a coarse retry-loop bucket
• document the allowed property style for retry attempt buckets

Evidence

• Sentry has 2 production microphone_start_timeout events on transcripted@1.1.44, both showing 11 start attempts on the same route shape
• PostHog has matching 1.1.44 dictation_start_failed events but lacked the retry-attempt dimension
• Candidate score: 91/100; meeting capture degradation lost because recent PR Classify meeting mic attenuation for issue #500 #901 already improved diagnostics and likely needs QA, not another tiny telemetry patch

Verification

• bash build-deps.sh --force
• bash build.sh --no-open
• bash run-tests.sh
• bash scripts/dev/agent-preflight.sh

[r3dbars]

verdict: no-findings-but-human

findings: none. The new analytics property is a coarse retry bucket, the raw start-attempt count stays out of the PostHog allowlist, and policy/docs coverage were updated.

actions: no fix pushed.
current_head_sha: 1c4e38a
fix_commit: none
checks: repo-hygiene pass; gh pr view; gh pr diff --patch; gh pr checks; git diff --check origin/main...origin/codex/nightly-telemetry-dictation-start-attempt-bucket-20260530; Observability/UI/Test docs reviewed.

manual_merge_handoff:
head_sha: 1c4e38a
verdict: no-findings-but-human
blockers: draft PR; human signoff for analytics/privacy payload surface
checks: repo-hygiene pass; PR metadata/diff/checks reviewed; diff-check clean; Observability/UI docs reviewed