Address review of projects-as-worktree (#34): git-repo check, doc comments, docs

[cloudmanic]

Follow-up to #34 (thanks again @dvic!) addressing the review notes on the ctrl+g "open highlighted project as a git worktree" feature.

Changes

Correctness / UX

• Removed dead code: the !filepath.IsAbs(dir) check in openProjectAsWorktree could never be true — filepath.Abs already returns an absolute path (or an error, already handled).
• Friendlier error for non-repos: openProjectAsWorktree now pre-checks that the directory is inside a git work tree via isInsideGitWorkTree. ctrl+g on a project whose working_dir isn't a git repo now returns not a git repository: … (opening as a worktree requires one) instead of herdr's raw worktree_create_failed. The check defers to herdr when git itself can't run, so a missing git never wrongly blocks.

Docs / consistency

• Doc comments added to the new functions/methods that lacked them (worktreeCreate, PluginConfig, configFilePath, loadPluginConfig, openProjectAsWorktree, resolveWorktreeBranch, updateBranch, promptWorktreeBranch, branchView), matching the rest of the codebase.
• README clarified: opening a project as a worktree fills its tabs from a matching worktrees/ worktree auto-layout — not the project's own [[tabs]] — and opens bare without one. This coupling was previously invisible.
• Fixed stale copied-header dates on the new files.

Tests

• projects_test.go covers isInsideGitWorkTree (plain dir vs git init).

Verification

go build, go test, and go vet all pass; the new git check test does a real git init and passes.

Note: the "tabs come from the worktree layout, not the project" behavior is documented rather than changed — falling back to the project's own tabs would risk double-applying against the worktree.created event handler. Happy to add that fallback separately if we want ctrl+g to work standalone without a worktrees/ file.