feat: define and enforce human approval gates for autonomous operations

[geoffjay]

feat: define and enforce human approval gates for autonomous operations

feat(pipeline): define and enforce human approval gates

• Create docs/planning/autonomous-pipeline-gates.md with the three-tier
  gate model: always-human, configurable (default shown), and always-autonomous
• Add tool_policy blocks to conductor.yml and worker.yml to deny force
  pushes, agent YAML edits, and issue/milestone deletions; ask before
  cargo add; conductor allows squash merges and label changes
• Add Human Approval Gates section to conductor system_prompt covering
  the always-human list, configurable gate table, temporary override via
  PR body OVERRIDE: lines, and escalation path to engineering room
• Update CLAUDE.md with abridged always-human list and reference to the
  full gate document

Closes #643

[geoffjay]

This change is part of the following stack:

• feat: define and enforce human approval gates for autonomous operations #705 ◀

_{Change managed by git-spice.}