workspace: warn against feature-first push on an empty repo

[gas2own]

Problem

On a repo with no commits, the first branch pushed silently becomes the default branch. If a headless agent follows a generic "always branch and open a PR" habit and pushes a feature branch first, there is then no main to open a PR against or deploy GitHub Pages from, and correcting the default branch requires repo-admin access the agent may not have.

Change

Add a concise warning before the first-save feature-push step. It tells the agent to check for main first and, when it is absent, have the repo initialized with a commit or ask an authorized operator to create main before continuing.

This is intentionally guidance rather than a mechanical guard: a direct main push may be blocked by the agent's host policy, a bespoke wrapper can be bypassed by raw git push, and CI runs too late to prevent the first pushed branch becoming default.

Validation

• python scripts/validate-skills.py
• git diff --check

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Validation (per the improve-skills eval workflow)

Measured through an asta_skills eval case rather than by inspection: allenai/asta-bench-private#230 adds empty_repo_bootstrap_no_feature_first_push + setups/empty_remote.sh, which stand up a scaffolded project whose remote has no branches and ask a headless agent to save+deploy.

• Intent metric checked_remote_for_main — did the agent check for main before pushing.
• Outcome guard no_feature_branch_on_empty_remote — the load-bearing metric; fails iff a non-main branch is pushed onto the empty remote (the exact lock-in this PR prevents).

The outcome metric command was exercised directly against three end-states and discriminates them: bare/pre-fix agent pushing a feature branch first → 0 (bug); fixed agent checking ls-remote and handing off → 1; permitted agent seeding main first → 1.

The full paired LLM arms (baseline origin/main skills vs this branch's skills, --epochs 3) need docker + inspect + ASTA_IMAGE, which weren't available where this was authored — flagging for a maintainer to run the arms per steps 3/5 of improve-skills and confirm the lift (baseline should fail no_feature_branch_on_empty_remote; this branch should pass).