feat: blanket @copilot delegation mode for implementation tasks #1677

Summary

When Plan Activate creates implementation task issues, users should be able to opt into having all coding tasks assigned to @copilot automatically — instead of distributing across cast members.

Motivation

Customers want the option of saying "let @copilot do anything coding-related." Today the Lead triages each issue individually against @copilot's capability profile. There's no blanket delegation mode.

Proposed Design

Policy flag

Add a config option (in team.md or config.json):

<!-- copilot-implementation-mode: all-coding -->

Or in config.json:

{
  "copilotImplementationMode": "all-coding"
}

Values: "triage" (default — Lead evaluates per-issue), "all-coding" (blanket delegation).

Plan Activate behavior

When copilotImplementationMode: "all-coding":

1. Task issues still get squad:{agent} labels for Squad tracking
2. Additionally, issues get assigned to @copilot via GitHub assignment
3. @copilot picks them up, creates copilot/* branches, opens draft PRs
4. Squad agents shift to review-only — Lead reviews PRs, Tester validates

Squad agent role shift

With blanket delegation active:

• Lead → reviews @copilot PRs, approves/rejects
• Tester → validates @copilot output against acceptance criteria
• Security → scans @copilot PRs for vulnerabilities
• Docs → updates documentation based on @copilot changes

Prerequisites

• copilot-setup-steps.yml must be configured in the repo
• @copilot must be on the Squad roster (squad cast-member copilot)

Relationship

• Extends the existing @copilot roster member support (.squad/templates/copilot-agent.md)
• Touches Plan Activate mode in workflows/squad.md
• Touches Lead triage logic in coordinator

Open Questions

• Should there be a per-epic override? (e.g., "all coding except security-sensitive tasks")
• Should @copilot PRs auto-request review from the Squad Lead?
• How does this interact with the Reviewer Rejection Protocol when @copilot's PR is rejected?