feat(session-audit): SessionEvent soft bridge — Sprint A Phase 1

[fivepanelhat]

Summary

Closes the observability gap between Weaver and Aether using Coastal-Alpine-Core SessionEvent (same vocabulary as Core #27 / Weaver #37).

Soft-import only — Aether remains fully usable when Core is not installed (Null store, zero behaviour change). Events are audit evidence only; Guardrails + ThreatModeler remain the sole decision authority.

CAT stamp: local-first JSONL (~/.aether/session_events.jsonl), no secrets in payloads, Te Mana Raraunga HITL evidence path.

Depends on

• Coastal-Alpine-Core#27 (merged — SessionEvent store v0.5.7)
• Weaver#37 (merged — parallel instrumentation)

Changes

• New aether/session_audit.py
  • Soft-import SessionEventStore / make_event / EVENT_TYPES
  • NullSessionEventStore fallback
  • install_session_event_hooks() — idempotent wrappers on:
    • start_task → session_start, prompt_received
    • call_tool → tool_call, tool_result
    • _execute_skill → skill_applied
    • _approve_if_needed → approval_required / approval_granted / approval_denied
    • run_react_loop / run_pipeline → session_end / error
• aether/orchestrator.py
  • Optional ctor flags: enable_session_events=True, session_event_path, tenant_id
  • Hooks installed at end of __init__ when enabled
• CHANGELOG.md — Unreleased Sprint A Phase 1 entry

Design constraints (preserved)

• No hard dependency on coastal-alpine-core in base deps
• No change to HITL / computer-use / injection-lock policy
• Payload rules: lengths, tool names, success flags only — no raw goals, no tool outputs, no secrets
• Fail-soft: emit failures never break the main loop

Testing & Validation

• Soft path: import and construct AetherOrchestrator() with no Core installed → Null store, normal operation
• With Core present: events append to local JSONL; same event types as Weaver
• Existing unit/smoke/computer-use suites unchanged by this PR

Te Tiriti & Cultural Alignment

• Local-first storage only; no silent exfiltration
• Supports kaitiakitanga of interaction evidence for HITL review
• No cultural content or PHI in this change

Checklist

• Soft-import + Null fallback
• HITL paths unchanged
• CHANGELOG updated
• No secrets in event payloads
• Optional follow-up: minimal tests/test_session_audit.py (Null + mock Core)

Follow-up

1. Merge this PR
2. Tag Core v0.5.7 if not already tagged
3. Sprint A Phase 2 — provider seams (shared across Core / Weaver / Aether)