feat(ce-work): codify worktree isolation for parallel subagent dispatch

[piiiico]

Problem

When /ce:work selects the "parallel subagents" execution strategy (Phase 1 Step 4), it dispatches multiple Agent tool calls for independent implementation units. Currently, the skill does not instruct the use of isolation: \"worktree\" on these Agent calls — subagents all operate on the same working directory, relying on a "non-overlapping files" heuristic.

This creates risks:

• Git operations can conflict between concurrent subagents
• Test runs can interfere with each other
• If the overlap assessment is wrong, file writes race

Evidence: it already works when the model chooses it

In a real session (Opus, 3 plans executed in parallel), the orchestrating agent independently decided to use isolation: \"worktree\" on each Agent call. The result:

• 3 worktrees created at .claude/worktrees/agent-{id}, each on its own branch
• Subagents ran fully isolated with run_in_background: true
• Orchestrator reviewed diffs from all 3 worktrees, then merged sequentially
• 3 merge conflicts arose on overlapping files — resolved cleanly by the orchestrator

The flow worked well, but it was emergent behavior — the model happened to use the parameter. A different model or context could skip it entirely.

Proposal

When the parallel subagents strategy is selected in Phase 1 Step 4, the skill should instruct:

1. Use isolation: \"worktree\" on each Agent dispatch
2. Use run_in_background: true for concurrent execution
3. After all subagents complete, review diffs from each worktree
4. Merge branches sequentially in dependency order, resolving conflicts
5. Clean up worktrees (note: .claude/worktrees/ should be in .gitignore to avoid accidental staging)

This makes the parallel strategy reliable rather than model-dependent.

Notes

• The serial subagents and inline strategies don't need this — they're sequential by definition
• PR feat: add /lfg-ext — external delegate LFG (Codex/Gemini/OpenCode in parallel worktrees) #223 (/lfg-ext) explored a similar pattern for external delegates but hasn't been merged
• The git-worktree skill's worktree-manager.sh is an alternative to the Agent tool's built-in worktree support, but isolation: \"worktree\" is simpler since the harness manages the lifecycle automatically