feat: add GitHub Copilot CLI backend

[Cannon07]

Summary

• Adds GitHub Copilot CLI as the third supported AI backend alongside Claude Code and OpenCode.
• Install writes a standalone .github/hooks/code-preview.json, matching Copilot's auto-discovered multi-file hook contract — sibling user hooks are never touched.
• Co-discovered fixes: *** Delete File: extraction in the post-tool path extractor (delete-diff tabs were lingering after mixed update+delete patches), and a stray empty hunk when GPT's *** Add File: shape coincided with @@.

What's included

Backend

• backends/copilot/{code-preview-diff,code-close-diff}.sh — translate Copilot's {toolName, cwd, toolArgs} payload to the normalized format consumed by bin/core-{pre,post}-tool.sh. Handles apply_patch (raw patch text), edit/str_replace, create/write, and bash. Noise tools (view, glob, report_intent, …) exit early before any RPC.
• lua/code-preview/backends/copilot.lua — install/uninstall + an is_our_config helper that grep-matches the adapter path. Uninstall refuses to delete a code-preview.json we didn't write; status reports "installed" only for files we own.
• :CodePreviewInstall/UninstallCopilotCliHooks commands; Copilot row in :CodePreviewStatus and :checkhealth.

Hardening

• arg() in both adapters binds the jq lookup key via --arg instead of interpolating into the program text.
• Pre-adapter skips cleanly on malformed payloads (empty path for edit/create, empty command for bash) rather than pushing empty-path diffs downstream.
• .gitignore narrowed from .github/ to .github/hooks/code-preview.json so workflows / FUNDING.yml / future repo-conventions files aren't hidden.
• Comments in bin/core-{pre,post}-tool.sh clarified to document CODE_PREVIEW_BACKEND's actual gating role and list all three backends.

Bug fixes

• bin/core-post-tool.sh: extractor regex now matches *** Delete File: alongside Update/Add — delete diffs now close on accept.
• bin/apply-patch.lua: lazy-create the hunk for *** Add File: so GPT's no-@@ shape captures content lines without producing a stray empty leading hunk when @@ does appear.

Tests

15 new tests, 45 total passing (bash tests/run.sh all):

• tests/backends/copilot/test_install.sh — install file layout, idempotency, sibling-file preservation, foreign-file uninstall guard.
• tests/backends/copilot/test_edit.sh — edit, create, bash rm, relative path, str_replace alias, noise-tool skip, malformed-payload skip.
• tests/backends/copilot/test_apply_patch.sh — Update, Add, mixed Update+Add+Delete with full close-cycle assertions.
• tests/core/test_post_tool_patch_paths.sh — unit-boundary regression for the patch-path extractor (stubs close_for_file).
• tests/backends/opencode/test_apply_patch.sh — adds the GPT-style no-@@ Add File parser case.
• tests/run.sh extended with core filter; all now picks up tests/core/ alongside tests/backends/.

Test plan

• bash tests/run.sh all — 45 tests pass locally
• Install/uninstall cycle in a real Copilot CLI session
• Mixed Update + Add + Delete patch — all diff tabs open on pre, all close on accept
• Manual edit/create/bash via Copilot with both GPT-5 and Claude Sonnet