feat(env): support standalone-repo worktrees in isolated envs

[jason10lee]

All Submissions:

• Have you followed the Newspack Contributing guidelines?
• Does your code follow the WordPress coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Isolated environments can already mount worktrees of monorepo plugins/themes (n env create --worktree <plugin>:<branch>). This extends --worktree to standalone checkouts that live outside the monorepo at repos/{plugins,themes}/<name> — private or customer-specific plugins, licensed extensions, etc. — so they can be branch-isolated in an env the same way monorepo projects are.

A standalone checkout is discovered by path (no registration). Its worktree is created from the checkout's own git history at worktrees/standalone/<repo>/<branch> and mounted serving-only: unlike a monorepo worktree, a standalone repo is not a pnpm workspace member, so it gets no /newspack-monorepo/... workspace mount. n env destroy and n env list understand the new mount shape and clean it up / display it.

This is additive — monorepo (tier-1) and legacy worktree handling are unchanged.

• bin/worktree.sh: n worktree add|remove <branch> --repo <name> for standalone checkouts; n worktree list enumerates standalone worktrees alongside workspace ones.
• bin/worktree-mounts.sh (new): a small, host-testable helper that emits tier-aware compose volume lines (monorepo → serving + workspace-member; standalone → serving only). Unit-tested.
• bin/env.sh: env create --worktree resolves standalone checkouts by path and mounts them tier-aware; the worktree-mount parser, env destroy, and env list gain standalone awareness.

How to test the changes in this Pull Request:

1. Drop a real standalone plugin checkout at repos/plugins/<name> (clone, unzip, or git worktree add).
2. n env create <env> --worktree <name>:<branch> --up — confirm the env comes up and the worktree is created at worktrees/standalone/<name>/<branch>.
3. Visit the env URL and confirm the standalone plugin is served from the worktree (edit a file in the worktree, n build <name>, see the change).
4. n env list — confirm the env shows the standalone worktree.
5. n worktree list — confirm a [<name>] section lists the standalone worktree.
6. n env destroy <env> — confirm the env, its worktree, and the branch are all removed.
7. Regression: repeat 2–6 with a monorepo plugin (--worktree newspack-plugin:<branch>) and confirm tier-1 both-path mounting and cleanup still work.
8. bash tests/worktree-mounts.test.sh — unit tests for the mount helper pass.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

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