fix: bound stalled git commands with actionable diagnostics

[mvanhorn]

Testing

• A normal git command that completes before the deadline returns the same trimmed output and ordinary nonzero-exit diagnostics as today.
• A git command run with an already-expired or deliberately short context terminates and returns a timeout-specific error rather than a generic exit error or an indefinite wait.
• The timeout diagnostic identifies the git subcommand and working directory and includes actionable stale-lock guidance for checkout/reset-style failures.
• IsHeadMergedIntoRef retains its special exit-code-1 meaning for an unmerged head while using the same bounded command execution path for hangs and other failures.

What Changed

Centralize subprocess construction in internal/git/git.go around exec.CommandContext and apply a conservative default deadline to every git command, including the merge-ancestor path that currently bypasses runGit; this protects fetch, worktree creation, checkout/reset/clean, and the repository-discovery commands without changing every caller. Preserve existing exit-code handling for commands that finish normally, but distinguish deadline expiry and return an error that names the stalled git operation and working directory, with concise hints for stale .git/index.lock, credentials, and network checks. Keep the timeout policy local to the git adapter rather than introducing a broad pool API or a helper used only by tests; the issue's immediate correctness requirement is a bounded failure, while the centralized duration remains a single production setting that can be exposed through configuration separately if maintainers want that additional surface.

Why

treehouse get and treehouse return ultimately launch git subprocesses through internal/git, but those commands have no deadline. A stale repository lock, blocked credential helper, or stalled network operation can therefore leave the CLI indefinitely at its setup banner. The maintainer confirmed the gap and invited a PR, and the issue has no assignee, competing PR, or closed-unmerged prior attempt.

Fixes #64