feat: opt-in observability — Sentry-protocol errors + Langfuse eval traces

[fazpu]

Owner-directed observability wiring, strictly opt-in via env (absent config = byte-identical behavior, nothing imported — proven by an import-purity test):

• Error tracking: REMEMBERSTACK_SENTRY_DSN (Sentry-protocol — works with Sentry/GlitchTip/Bugsink) initializes in the selfhost api/worker/setup entrypoints; the worker exception path captures with stage/lane/processing_id tags. Metadata only — never prompt/completion text (allowlisted before_send).
• Langfuse eval tracing: standard LANGFUSE_* vars; the LoCoMo answer/judge stages emit one trace per question with per-call spans (model, usage, cost, latency, verdicts). Observer-only: the protocol fingerprint (prompts/schemas/catalog/manifests) has zero diff, asserted against baseline.
• Observability never alters production control flow (review round): all sink/tracer failures are best-effort-isolated — only the authoritative local sink may raise; regression tests include a raising Sentry sink through run_one and a raising tracer through answer/judge with identical outputs.
• Optional dependency extra observability; compose/.env.example/docs wired. Instance provisioning (Langfuse self-host vs cloud; GlitchTip vs Betterstack) is an infra decision tracked separately.

Review: Grok REQUEST-CHANGES (2 MAJOR on control-flow isolation, 3 MINOR, 2 NIT) → all applied → verified. Final review by Claude. Fast gate local (130 passed); full matrix in CI.

• I have read and agree to the RememberStack Contributor License Agreement v1.0.

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/rememberstack/adapters
__init__.py
sentry.py					119-125, 157, 166, 169, 172
src/rememberstack/adapters/selfhost
__init__.py
telemetry.py					58, 63-68
src/rememberstack/profiles
selfhost.py					115, 159-166, 171-172, 314-317, 327-336, 540-542, 572-579, 587, 595, 599, 610
Project Total

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