feat(plugins): add ocsf-audit — OCSF API Activity emitter with attestation chain

[Levaj2000]

Adds builtins/plugins/ocsf-audit — a CMF audit plugin that emits each dispatched
request as an OCSF API Activity event (class 6003, ai_operation +
security_control profiles) instead of a free-form JSON line. Near-twin of
audit-logger: same observation-only, always-allow contract, same factory + hook
wiring; only the record shape differs.

What it does

• Maps CMF Message + Extensions onto OCSF: ai_agent, ai_model,
  message_context, delegation, actor/user, tool/resource coordinates.
• Activity ids follow API Activity's real enum (readOnlyHint: true tools,
  resources, prompts → 2 Read; other tool calls → 99 + "Invoke Tool";
  completions → 99 + "Completion"). The passive post-hook stream carries
  action_id: 3 (Observed) / disposition_id: 17 (Logged).
• Optional tamper-evident attestation chain with predecessor binding —
  entry_hash = sha256(canonical{chain_uid, event, prev_entry_hash}) over JCS-style
  canonical bytes — independently recomputable from the emitted JSON, no shared
  secret. DSSE signing is a seam (stub) consuming the same binding bytes.
• CMF fields with no OCSF home yet (stop_reason, mcp.*, framework.*, monotonic
  labels, workload identity) ride under OCSF unmapped, so the open schema gaps stay
  self-documenting; upstream OCSF issues for them are being filed.

Scope / deliberate limits

• Registered as a feature-gated builtin (ocsf-audit), not in default features.
• Deny/modify records (action_id 2/4) need the framework to surface its decision
  to a plugin — per the production-readiness plan, not attempted here.
• Chain head is in-process (one chain per instance, reset on restart); durable /
  replica-safe chaining and checkpoint signing are follow-on work.

Validation: cargo build/test -p cpex-plugin-ocsf-audit (13 tests),
cargo clippy --all-targets clean against the workspace lint wall, cargo fmt
clean, cargo build -p cpex-builtins --features ocsf-audit clean.
SAMPLE-OUTPUT.md holds the deterministic example output.

Follows the plan in ocsf-plugin-production-plan.md (P0 schema-validity items are
in; P1+ tracked there).