refactor(switch): unify the picker and argument-path switch pipelines

[max-sixty]

Summary

wt switch <branch> and the interactive picker (wt switch with no argument) each ran the same switch sequence as separate, parallel code. #2845 made the two paths behave identically; this makes the code identical too — a single SwitchPipeline that both entry points build and .run().

SwitchPipeline::run (in src/commands/worktree/switch.rs) owns the whole sequence: the bare-repo worktree-path fix-up, pre-switch hooks, source-identity capture, plan_switch → approve_switch_hooks → validate_switch_templates → execute_switch, output, background hooks, and --execute. Each caller now only resolves a branch identifier and loads config. The picker-vs-argument differences (--execute, the shell-integration offer, source-identity capture) are struct field values, not divergent branches.

Bug fixed

The duplication hid a real bug: the picker passed yes = true to run_pre_switch_hooks, auto-approving project pre-switch hooks without a prompt — unapproved code from a freshly cloned .config/wt.toml running silently. Every other hook the picker runs (post-switch, pre-create, post-create) already went through the approval prompt. With one shared run_pre_switch_hooks call gated by the pipeline's single verify/yes pair, the picker (which has no --yes) now prompts for project pre-switch hooks like wt switch <branch> does — and the two paths can't drift on hook approval again.

Reviewer notes

• One intentional reorder on the argument path: offer_bare_repo_worktree_path_fix now runs before run_pre_switch_hooks (the picker already used this order). The fix only mutates worktree-path config, which pre-switch hooks never read — behavior-neutral.
• capture_switch_source runs inside run() after pre-switch hooks — the same relative position as the old run_switch.
• Eight now-internal helpers were narrowed from pub/pub(crate) to private; worktree/mod.rs re-exports trimmed accordingly.

Testing

Pure refactor — the existing test_switch_*, test_switch_format_json_*, and test_switch_picker_* suites cover behavior preservation. test_switch_picker_pre_switch_hook_requires_approval is a new regression test for the bug fix (verified to fail under the bug).