feat: open existing worktree on duplicate pr checkout

[ppn26]

When wt pr checkout targets a branch that already has a worktree, open it
instead of failing with a fatal git error. Fetches latest changes, refreshes
PR cache, and runs hooks with action="open".

Mirrors the behavior already implemented for wt checkout in c730458.

Breaking Changes

• wt pr checkout <number> no longer errors when the branch already has a
  worktree — it opens it instead.

Test Coverage

• Integration tests added: 1

🤖 Generated with Claude Code

[raphi011]

Review Feedback

Hook action should stay ActionPR, not ActionOpen

wt pr checkout is semantically a PR action — if the user just wanted to open an existing worktree they'd use wt co <branch>. Hooks configured with on=["pr"] should still fire when reopening a PR worktree. Keep action = hooks.ActionPR for all pr checkout paths.

Test coverage

The test calls findWorktreeForBranch and openExistingWorktree directly, which are already covered by checkout tests. The new code unique to this PR (the else if placement, hook action, output message selection) isn't exercised. Consider verifying stdout output ("Opened worktree" vs "Created worktree") at minimum.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/wt/pr_cmd.go	0.00%	12 Missing ⚠️

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