feat: add pre-switch hook

[max-sixty]

Summary

• Add pre-switch hook that runs before wt switch does anything — before branch validation, planning, or worktree creation
• Runs in both the direct switch path and the interactive picker path
• Includes a fetch-if-stale example (check FETCH_HEAD age, fetch if >6 hours)
• Add orienting sentence in hook docs pointing to post-start as the most common starting point

Closes #1085. Thanks to @jdb8 for reporting.

Test plan

• test_user_pre_switch_hook_executes — hook runs in source worktree before switch
• test_user_pre_switch_failure_blocks_switch — failing hook aborts --create
• test_user_pre_switch_skipped_with_no_verify — --no-verify skips hook
• All 1075 tests pass, all lints pass

This was written by Claude Code on behalf of @max-sixty

[worktrunk-bot]

Clean feature addition — the hook plumbing, docs, CLI wiring, and tests all follow the established patterns well.

[worktrunk-bot]

yes: true here auto-approves project pre-switch hooks without prompting, while approve_switch_hooks on line 320 uses yes: false (which prompts). This means project-level pre-switch hooks silently auto-approve in the picker, but post-switch/post-create project hooks still prompt — feels inconsistent.

Suggested change

	run_pre_switch_hooks(&repo, config, true)?;
	run_pre_switch_hooks(&repo, config, false)?;