feat(gh-aw): /squad cast — iterate squad composition via PR/issue comments #1617

/squad cast & /squad retire — Conversational Squad Iteration

Part of #1612 (Squad Agentic Workflows epic).
Depends on: #1614 (Cast mode — the initial team must exist first)

Overview

The key UX innovation: squad composition becomes a PR-native conversational flow. Users add, remove, and modify team members through natural language comments on issues and PRs.

Command Patterns

• /squad cast a frontend accessibility specialist — add a new member
• /squad cast rename Hockney to focus on performance — modify existing member
• /squad retire McManus — remove a member
• /squad status — report current team state

Behavior on PRs vs Issues

Agent Intelligence

• Uses Squad's full casting intelligence for new members:
  • Universe selection (picks appropriate name universe)
  • Name allocation (avoids conflicts with existing cast)
  • Charter generation (tailored to the specified role/specialty)
• For retirement: removes agent charter, updates routing, updates casting registry
• For modifications: regenerates charter with new focus, preserves name/history

Examples

User: /squad cast a mermaid diagram specialist
Agent: → Creates "Vega" (from data-viz universe), writes charter focused on
        Mermaid diagram generation, updates routing.md, pushes to PR branch

User: /squad retire the tester
Agent: → Identifies FIDO (testing role), removes charter, updates team.md
        and routing.md, pushes commit

User: /squad cast rename Hockney to focus on performance
Agent: → Updates Hockney's charter to emphasize performance work,
        adjusts routing rules accordingly

Acceptance Criteria

• /squad cast {description} adds a new team member with full casting intelligence
• /squad retire {name/role} removes a member cleanly
• /squad cast rename/modify updates existing member's focus
• /squad status reports current team composition
• On Squad PRs: changes are pushed as commits to existing branch
• On issues: a new PR is opened with the change
• Name allocation avoids conflicts with existing cast
• Charter generation is tailored to the specified specialty
• Routing and team.md are updated to reflect changes
• Casting registry is kept in sync