docs: stop claiming Provena blocks context before the LLM

[yuee3]

What

Stop presenting Provena as an in-line gate that blocks bad context before it reaches the LLM. Documentation accuracy only — no runtime change.

1. Rewrite the Architecture ASCII diagrams in README.md and docs/index.md so Provena is not drawn as ContextTrail ---> LLM Context Window. The replacement conveys: sources are observed/logged into the trail (passive by default); optional policies may WARN/BLOCK after persist; the LLM path is the application's, not Provena's.
2. Replace the README example comment that claimed pre-LLM blocking (# Blocked: freshness_check failed — STALE context blocked before reaching LLM) with wording that says the PolicyViolation is raised after the record was written, and that with @track the wrapped call has already finished.
3. Soften the Policy Enforcement table BLOCK row so it no longer implies call-site gating: raise PolicyViolation after persist; default trails with policies=None only observe/log.
4. Keep the already-correct "blocked entries are always persisted" line — it now reads as the compliance story instead of competing with "before reaching the LLM".

Why

Closes #199. New users reading the hero README / docs landing page would believe Provena sits in front of the LLM. In src/provena/trail.py, _log_internal appends (or buffers) the record, then _enforce_policies; empty PolicyEngine always ALLOWs; @track calls func first and only then _track_result / _log_internal. The gate language created false security expectations.

Narrower than #56 and not a duplicate — this is specifically the false "blocked before the LLM" claim.

How checked

• Read src/provena/trail.py (log / _log_internal, _enforce_policies, @track) and confirmed persist-then-policy order and that @track runs the wrapped body to completion before logging.
• Matched every "What to do" item in README and docs/index overstate blocking context before the LLM #199 to a concrete edit.
• Documentation-only. No code, config, or project-context/learning-guide.md touched.

Scope

• README.md — Architecture diagram; E2E example comment; Policy Enforcement BLOCK row
• docs/index.md — Architecture diagram