feat(dev): fix dot-worktree resolution, unique task files, and --agent/-a UX

[basnijholt]

Summary

• allow dev agent . and other dev commands to resolve the current worktree via find_worktree_by_name
• write prompts to unique .claude/TASK-{timestamp}-{hex}.md files to avoid parallel-launch collisions
• use mkstemp for unique prompt wrapper scripts
• make --agent/-a the primary selector for dev agent, while keeping --with-agent as a deprecated alias
• update help text, docs, and skill guidance to match the new behavior

Testing

• uv run pytest tests/dev/test_cli.py tests/dev/test_launch.py tests/dev/test_worktree.py

[basnijholt]

[BUG] . resolves to the main checkout for nested worktrees

_find_worktree_for_cwd() currently returns the first worktree whose path prefixes the current directory. git worktree list reports the main checkout first, so when a worktree lives under the repo root (for example .worktrees/feature), the main repo matches before the nested worktree does.

That makes the new . behavior target the wrong checkout for the exact dot-worktree layout called out in this PR: dev agent ., dev path ., dev editor ., and dev rm . will all resolve to the main repo instead of the nested worktree.

Minimal repro:

• main repo: /repo
• worktree: /repo/.worktrees/feature
• cwd: /repo/.worktrees/feature/src
• current result: /repo
• expected result: /repo/.worktrees/feature

Please pick the most specific matching worktree path instead of the first match, and add a regression test that uses a nested .worktrees/... layout.

[MINOR] docs/commands/dev.md still describes the old task-file behavior

The paragraph at docs/commands/dev.md:748 still says .claude/TASK.md reflects the most recent launch. After this patch each launch writes a unique TASK-{timestamp}-{hex}.md, so that note is now stale.