feat: add worktree creation/lifecycle to coordinator spawn flow

[joniba]

Summary

Closes #525. Adds worktree creation and lifecycle management to the coordinator spawn flow. Previously, Squad could detect and read worktrees but never created them — all branch-creation paths hardcoded git checkout -b, breaking parallel multi-agent execution.

Changes

1. ralph-commands.ts — New worktree commands

Added createWorktree and removeWorktree to the RalphCommands interface and all 3 platform adapters (GitHub, ADO, Planner):

• createWorktree: git fetch origin {baseBranch} && git worktree add {worktreePath} -b {branchName} origin/{baseBranch}
• removeWorktree: git worktree remove {worktreePath} && git worktree prune

2. squad.agent.md — Worktree-aware spawn flow

• Parallel Fan-Out section: Added step 3 (create worktrees for parallel issue work before spawning agents)
• Worktree Lifecycle section: New section documenting decision logic (single agent = checkout, 2+ agents = worktrees), pre-spawn creation, and post-merge cleanup
• Spawn template: Added WORKTREE_PATH block — tells agents they're in a dedicated worktree and must not run git checkout
• Issue → PR → Merge Lifecycle: Updated to describe both single-agent (standard) and parallel-agent (worktree) paths; fixed reference to issue-lifecycle.md

3. issue-lifecycle.md — New template (was missing)

Created the template that was referenced twice in squad.agent.md but never existed. Contains:

• Decision logic table (when to use checkout vs worktree)
• Single-issue workflow (standard checkout -b)
• Parallel-issue workflow (worktree-based) with coordinator pre-spawn, spawn prompt ISSUE CONTEXT block, per-worktree agent workflow, and post-merge cleanup
• PR review handling and merge commands

4. git-workflow SKILL.md — Worktrees as default for parallel work

• Restructured from flat workflow to Single Issue + Multiple Issues in Parallel subsections
• Made worktrees the default strategy for 2+ simultaneous issues (was a separate "advanced" section)
• Updated naming convention to ./worktrees/squad-{issue-number} (was ../{repo-name}-{issue-number})
• Added coordinator pre-spawn responsibility (coordinator creates worktrees, not agents)
• Updated per-worktree agent workflow to document WORKTREE_PATH and no-checkout rule

5. Tests

• ralph-commands.test.ts: Added createWorktree and removeWorktree to COMMAND_KEYS, plus 2 new cross-platform tests verifying all platforms support worktree commands with correct placeholders
• platform-adapter.test.ts: Added placeholder token assertions for createWorktree and removeWorktree

Test Results

All 134 tests passing (ralph-commands.test.ts + platform-adapter.test.ts).