Add multi-channel webhook notifications (Slack/Discord/Teams/raw)

[JE-Chen]

Devex batch (from multi-agent web research; verified absent). Full layers + tests + EN/Zh v50 docs + README.

Feature (utils/notify_channels, pure-stdlib)

• notify_webhook / WebhookChannel: notify was desktop-toast only and ChatOps shipped Slack only — this sends to Slack / Discord / Microsoft Teams / raw webhooks, building the transport-shaped payload (Slack & Teams MessageCard use text, Discord uses content) and POSTing via the egress-guarded HTTP client. WebhookResult carries ok/status/transport.
• Transport injectable (poster arg or set_default_poster) → unit-tested with no network.
• Executor AC_notify_webhook; MCP ac_notify_webhook; Builder under Tools.

Verification

• 9 tests pass (slack/discord/teams/raw payload shapes, ok-by-status, channel reuse, executor round-trip via default poster, wiring, facade); ruff clean; radon no CC≥C; bandit clean; PySide6-free.
• Stacked on cloudevents (Add outbound CloudEvents emitter #257); rebased cleanly onto dev.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 36 complexity · 0 duplication

Metric	Results
Complexity	36
Duplication	0

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud