chore: replace stale agent docs with a headless-boundaries skill

[esmuellert]

Summary

The agent docs described an E2E workflow that no longer exists. scripts/nvim-e2e.lua and tests/e2e/ were removed in 31510a9, which made the documented scenario format, the runner invocation, and the entire e2e.* helper API fiction. An agent following those docs would fail on the first command.

This replaces them with a skill that documents what is observable in headless Neovim rather than an API surface that rots on the next refactor.

Changes

Removed

• .github/skills/nvim-e2e-workflow/ (360 lines) — referenced the deleted runner; its 3 examples used the git("add . && git commit") chaining that the docs elsewhere explicitly warn against
• .github/agents/codediff-developer.agent.md (220 lines) — 66 lines of it documented e2e.* helpers that do not exist; module tree had drifted
• CLAUDE.md — Claude Code is no longer used; the file only held symlink setup

Added

• .agents/skills/nvim-headless/SKILL.md (101 lines) — headless boundaries, the three driving styles, and known traps

Fixed

• AGENTS.md pointed at .github/copilot-instructions.md files that never existed
• Two spec comments blamed a missing UI for WinScrolled not firing

Why .agents/skills/

.github/skills/ is read only by Copilot. .agents/skills/ is read by Copilot, pi, Codex, Cursor and Gemini CLI, with no config file or symlink. Verified locally that both copilot -p and pi -p discover the skill from a clean checkout.

On the WinScrolled comments

The two comments claimed headless has no UI, therefore WinScrolled does not fire. Measured against a real UI in a pty, both behave identically:

driving style	headless	real UI
synchronous in one chunk	0	0
synchronous + vim.wait	0	0
defer_fn chain	5	5
nvim_exec_autocmds	5	5

What matters is that synchronous spec code never returns to the main loop, so the deferred event is never dispatched. The nvim_exec_autocmds workaround was correct; only the stated reason was wrong. This matters because it tells an agent it is not blocked by headless — it can switch driving style instead of giving up.

Testing

• tests/ui/explorer/explorer_scroll_independence_spec.lua — 3 passed
• tests/ui/view/view_spec.lua — 17 passed
• Every repo-internal path referenced by the remaining agent docs verified to exist
• No dangling references to CLAUDE.md, codediff-developer, nvim-e2e-workflow, or .github/agents

Docs only; no production code touched.