feat: Channels integration for asynchronous gate approval and stuck alerts

[sriumcp]

TL;DR

At HUMAN_DESIGN_GATE / HUMAN_FINDINGS_GATE, push a summary card to a configured Channel (Slack / Telegram / iMessage / webhook). Approve from your phone. Resume continues automatically. Same channel surfaces stuck-detection alerts (#7).

Why this matters

Long campaigns are run unattended with --auto-approve. When you do want a human in the loop you need to be at the terminal at the right second; otherwise gates auto-approve indistinguishably from a real approval. Channels is purpose-built for cross-device human-in-the-loop.

What's already shipped

• feat: Phase 4.5 — Code-access agents, simplified campaigns, gate summaries #29 added gate summaries.
• Improve human gate UX and replace feedback.md with structured feedback.json #33 improved gate UX and replaced feedback.md with structured feedback.json.
• This issue routes those summaries to off-terminal surfaces.

Proposed approach

1. Add channels: to campaign.yaml — list of channel configs (slack webhook URL, telegram bot, etc.).
2. At each human gate, instead of (or in addition to) blocking the terminal:
   • POST the existing gate_summary_<phase>.json (rendered as markdown) to each channel.
   • Include "approve" / "reject" / "abort" reply tokens.
3. Wait for a reply (with a configurable timeout that falls back to the campaign's existing auto-approve policy).
4. Reuse the same channel surface for stuck-detection alerts emitted by Phase 2: Templates + orchestrator skeleton #7.

Acceptance criteria

• A Slack-configured campaign posts the gate summary to the configured channel.
• Replying with "approve" advances the campaign without terminal interaction.
• If no reply arrives within the timeout, behavior falls back to the existing --auto-approve semantics.

Out of scope

• Building a UI for replies — use whatever each channel provides natively (Slack interactive messages, Telegram inline keyboards, plaintext fallback).

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Part of #120.

[sriumcp]

Closed by #153 (squash-merge 3e69b85, merged into reflective 2026-05-24). The merged commit message contains an explicit Closes #130. GitHub auto-close didn't fire because the PR targets reflective rather than the repo's default branch.