feat(protocol): add EGRESS as a sixth categorization (Phase 2c-2d)

[jinhongkuan]

Summary

Resolves the open question surfaced in #505: egress.deliver was on the namespace-invariant allowlist because the 2c-1 categorization committed to five prefixes and didn't include outbound delivery.

Option 1 chosen (with @jinhongkuan): add EGRESS_PREFIX = "egress." as a sixth category symmetric with IngestAdapter ↔ EgressAdapter in the Protocol surface.

Egress is structurally distinct from write.* — it pushes NotificationEvent to channel adapters, not rows to the ledger. Conflating them at the wire level would force adapter authors to reason about which write.X mutates state vs which fans out a notification. Cleaner mental model under its own prefix.

What changed

• protocol/categorization.py:
  • new EGRESS_PREFIX constant
  • Category.EGRESS enum value
  • _PREFIX_BY_CATEGORY mapping entry
  • new egress_tool() decorator function
  • docstring updated 5 → 6 categories
• protocol/__init__.py — export egress_tool, EGRESS_PREFIX
• tests/test_protocol_categorization.py:
  • _PREFIX_BY_CATEGORY mapping extended
  • test_all_five_categories_smoke → test_all_six_categories_smoke
  • new test_egress_tool_rejects_mismatch (parallel to existing read_tool mismatch test)
• tests/test_protocol_namespace_invariant.py:
  • egress.deliver removed from WIRE_METHOD_ALLOWLIST (now matches proper EGRESS_PREFIX)
  • EGRESS_PREFIX added to _CATEGORIZED_PREFIXES tuple

Timing rationale

Egress dispatch is a stub today — no hosted-mode adapter speaks it yet. Renaming-the-namespace cost is zero now. Once 2c-3 spins up real egress dispatch and the daemon becomes a real process, the wire name becomes load-bearing and a hosted-mode rename means coordinated client+server release. Cheaper to settle the contract while no one's compiling against it.

Test plan

• pytest tests/test_protocol_categorization.py — 9 passing (was 8 + new test_egress_tool_rejects_mismatch)
• pytest tests/test_protocol_namespace_invariant.py — 3 passing (allowlist now empty)
• Full protocol suite — 37 passing (was 36)
• ruff format --check . && ruff check . clean
• mypy protocol/ clean
• CI green on dev

Known limitation

Bicameral preflight unavailable this session (MCP server disconnected). Will validate against the ledger before opening 2c-3.

Plan refs

• Arc: thoughts/shared/plans/2026-05-22-daemon-as-process-arc.md
• Predecessors merged: feat(protocol): read/write tool categorization (Phase 2c-1) #500 (categorization), feat(protocol): read.* surface — typed RPC for history + usage_summary (Phase 2c-2) #501 (read surface), feat(protocol): write.* telemetry surface — feedback + skill_begin/end (Phase 2c-2b) #503 (telemetry writes), feat(protocol): rename ingest.* + grounding.* to match categorized namespace (Phase 2c-2c) #505 (namespace migration)

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