docs: add a skill for AI coding agents, compile-checked in CI

[vyncint]

A skill for coding agents (Claude Code, Cursor, Copilot) that write tests with termlens, at skills/termlens/SKILL.md, plus a CI guard that compiles every Rust block in it against the crate.

What is in the skill

• The model in sixty seconds, then twelve golden rules — each with the reason, not just the instruction: no sleep (and send_after as the one sanctioned delay), snapshot_after for whole-screen snapshots, one predicate per instant, bin! for own binaries, the 2..=1000 geometry floor and ceiling, the two coordinate orders, finish the process and assert !alternate_screen(), wait_frame only for applications that emit synchronized updates, termlens::Result<()> tests, snapshot the Screen not its text, the hermetic environment, cells and wide characters.
• Four recipes, each a complete integration test: CLI --help snapshot with exit code; ratatui navigation and clean quit; overriding size, environment and deadline; targeted find/cell/style/rect_text/cursor assertions including CJK.
• A failure-triage table quoting the crate's actual error strings, an API cheat sheet, a pitfalls table ("you are about to write X → write Y"), an insta pairing section and a finishing checklist.

What was measured before writing it

• ratatui 0.30.2 + crossterm emits no DEC 2026 synchronized updates (repaints() stays 0 after a draw), so the skill sends agents to snapshot_after, and names the exact timeout message wait_frame produces otherwise.
• The four recipes were extracted mechanically from the skill and run against a real ratatui 0.30.2 binary named myapp (list with highlight, j/k/q, --help, NO_COLOR): 5 tests, green 7 runs in a row, snapshots stable.
• The instant-exit --help case ran 100 times with both wait orders on an idle machine with no loss; the skill still recommends waiting on the last line first, and says why (the macOS teardown race under load, from DESIGN §2).
• A fresh claude -p session in this checkout reports the skill available and quotes its description, so the .claude/skills/termlens symlink is discovered.

The guard

.github/scripts/check-skill-snippets.sh writes each ```rust block to a scratch consumer owning a stub myapp and runs cargo check --tests; on failure it prints which snippet is which line of the skill. Verified to fail on a renamed method. New skill job, in required-green. It compiles and does not run — the stub is fn main() {}.

Not done, deliberately

• No ratatui fixture in this repository (the binary the recipes ran against lives outside it); the guard therefore compiles rather than runs the recipes.
• The skill installs as a single file, so there is no references/ split; at 450 lines it is dense on purpose.
• Nothing in the skill references this repository's issue tracker or planning documents.