chore(skills): the tree's extension surfaces ship empty

[Max17190]

Why

A file under .agents/ or .openmax/ is installed capability that every session working in this repo pays frozen-prompt bytes for on every request, not an example. That principle removed the shipped plan mode in #222, but the delegate and parallel-explore skills predate it: they landed the same day #37 removed the built-in task subagent, as its replacement, and survived as a grandfathered exception. Measured with openmax --spec usage, they cost 530 chars (~132 tokens) of frozen prefix per request in this repo.

Summary

• Remove the four tracked files under .agents/skills/ (the recipes are retained outside the repo with the measurement rigs).
• AGENTS.example.md's nested-agents row now says to ask the agent to author a delegate skill when the project wants one, instead of naming pre-installed files.
• New guard the_extension_surfaces_ship_empty (prompt.rs) reads the repository index via git ls-files and fails when any tracked file returns to either surface; it skips only when git or a checkout is absent (a packaged crate), and fails on any other read problem.

Test Plan

• The guard was run against the pre-removal tree first and failed, listing the four files; after removal it passes.
• cargo test --workspace: 842 passed, 0 failed.
• cargo clippy --workspace --all-targets: zero warnings.
• Frozen-prefix cost measured in a fixture project with the same binary: 8056 chars before, 7526 after, a 530-char (~132 token) saving on every request.

Greptile Summary

The repository now recognizes packaged source trees by the absence of .git metadata rather than parsing Git’s localized error text. The extension-surface guard skips correctly in a source-tree copy, passes in this checkout, and rejects a tracked file placed under a protected extension directory.

Merge-safe: yes.

Confidence Score: 5/5

No blocking failure remains.

The packaged-tree behavior was exercised under a non-English locale and no longer depends on Git diagnostic text; the repository guard also accepted the current checkout and rejected a controlled tracked extension file.

T-Rex Logs

What T-Rex did

• The T-Rex run executed the uploaded shell harness against three configurations to validate the extension-surface guard policy.
• In the German locale with no Git present, the source-tree test printed a no-checkout skip and did not invoke a fake Git.
• The reviewed checkout passed with no protected files listed by the Git index.
• The controlled checkout failed at crates/core/src/prompt.rs:733 when naming the tracked extension file.
• Overall, the tests show the guard is locale-independent and enforces the intended tracked-file policy.

_{Ran code and verified through T-Rex}

_{Reviews (3): Last reviewed commit: "fix(core): the packaged-tree skip is str..." | Re-trigger Greptile}