feat(observability): wire Phase B causal audit boundaries

[RuroninMugen]

Summary

Adds the production-only Phase B causal observability layer for buzz-acp.

This PR introduces a local, metadata-only, fail-open JSONL audit sink and wires seven production observation boundaries:

• TransportPublished
• TransportAccepted
• EventReceived
• FilterDecision
• QueueDecision
• WakeDecision
• AcpSessionStarted

Safety / behavior

• auditing is disabled by default
• no blocking filesystem I/O occurs on hot paths
• bounded channel + try_send prevents backpressure from affecting business logic
• filesystem/writer failures fail open
• audit payloads exclude raw prompts, message bodies, relay text, API keys, private keys, and auth tokens by construction
• no dependency changes

Validation

Validated on current upstream main before push:

• rustup run 1.95.0 cargo check -p buzz-acp — PASS
• rustup run 1.95.0 cargo clippy -p buzz-acp --all-targets — exit 0, warnings only
• rustup run 1.95.0 cargo test -p buzz-acp — 810 passed, 0 failed

All five commits carry DCO Signed-off-by trailers.

Scope / known limitations

This PR is intentionally production-only. The separate Phase 5B regression-test commit is not included here.

Known limitations:

• ResponsePublished is declared in the schema but not yet wired to a production emit site.
• RecipientEvidenceObserved is declared in the schema but not yet wired to a production emit site.
• TransportPublished / TransportAccepted describe internal buzz-acp WebSocket transport observations; they are not proof of real outbound agent handoff/chat publication through buzz-cli.
• correlation_id_for is currently unused.
• fail-closed FilterDecision emitted-payload coverage and B7→B8 cross-record correlation remain follow-up test work.

The original combined PR #6559 remains untouched; this PR replaces only its production portion with a clean, DCO-signed branch based on current upstream main.