Feature request: configurable branch naming for subagent worktrees

Problem

When using isolation: "worktree" on the Agent/Task tool, Claude Code generates branch names like worktree-agent-a5f404ec — these are auto-generated hashes that are meaningless when they show up in PR lists on GitHub.

When dispatching multiple parallel agents that each create PRs, the PR list becomes unreadable:

worktree-agent-a5f404ec  chore(tests): remove __init__.py files
worktree-agent-ad5a91a1  refactor(tests): parametrize serial number tests
worktree-agent-aa2385bd  refactor(tests): parametrize cloud platform tests

Current workaround

After an agent completes, manually push to a semantic branch name before creating the PR:

git push origin "origin/worktree-agent-XXXX:refs/heads/refactor/descriptive-name"

This is tedious when dispatching 6+ agents in parallel.

Proposed solution

Allow the Agent/Task tool to accept an optional worktree_branch (or similar) parameter that controls the branch name when isolation: "worktree" is set:

{
  "description": "Parametrize serial number tests",
  "isolation": "worktree",
  "worktree_branch": "refactor/parametrize-serial-number-tests"
}

Alternatively, derive a slug from the description field (e.g., "Parametrize serial number tests" → agent/parametrize-serial-number-tests).

Additional context

• The WorktreeCreate hook receives the auto-generated name but there's no way to influence what Claude Code passes as that name for subagent worktrees.
• The description field on the Agent tool already contains a human-readable summary that would work well as a branch name slug.