refactor: rename to code-preview, split backend modules, add layout + visible_only tests (#13)

[Cannon07]

Summary

• Rename plugin from claude-preview to code-preview across all Lua modules, shell scripts, TypeScript, commands, and tests
• Split monolithic hooks.lua into dedicated backends/claudecode.lua and backends/opencode.lua modules
• Relocate adapter scripts into backends/claudecode/ and backends/opencode/ directories
• Add deprecated aliases for all old :ClaudePreview* commands so existing users aren't broken
• Add tests for diff layout variants (tab / vsplit / inline) and visible_only mode

Changes

Rename: claude-preview → code-preview

• lua/code-preview/ (renamed from lua/claude-preview/) — all modules updated
• All require("claude-preview.*") → require("code-preview.*") throughout Lua, shell scripts, and TypeScript
• Commands renamed: :CodePreviewInstallClaudeCodeHooks, :CodePreviewUninstallClaudeCodeHooks, :CodePreviewCloseDiff, :CodePreviewStatus, :CodePreviewToggleVisibleOnly, :CodePreviewInstallOpenCodeHooks, :CodePreviewUninstallOpenCodeHooks
• Deprecated aliases — all old :ClaudePreview* commands still work but emit a deprecation warning pointing to the new command name
• bin/nvim-send.sh — mktemp prefix updated to code-preview-nvim-cmd
• tests/minimal_init.lua and all spec files updated

Backend restructure

• lua/code-preview/backends/claudecode.lua (new, replaces hooks.lua Claude Code logic) — install(), uninstall(), dual-marker uninstall ("code-preview" and "claude-preview") so users with old hooks can cleanly uninstall after upgrading
• lua/code-preview/backends/opencode.lua (new, replaces hooks.lua OpenCode logic) — install(), uninstall()
• lua/code-preview/hooks.lua — deleted (split into the two backend modules above)
• backends/claudecode/code-preview-diff.sh (moved from bin/claude-preview-diff.sh) — sets CODE_PREVIEW_BACKEND=claudecode, execs bin/core-pre-tool.sh
• backends/claudecode/code-close-diff.sh (moved from bin/claude-close-diff.sh) — execs bin/core-post-tool.sh
• backends/opencode/ (moved from opencode-plugin/) — index.ts env var updated CLAUDE_PREVIEW_BACKEND → CODE_PREVIEW_BACKEND; package name updated to code-preview-opencode
• bin/core-pre-tool.sh / bin/core-post-tool.sh — all requires and env var references updated

Health check

• lua/code-preview/health.lua — dual-marker hook detection: reports OK for new marker, warns with migration hint for legacy claude-preview marker, and fails if neither is found

Tests

• tests/plugin/diff_lifecycle_spec.lua — new "diff layouts" describe block with 3 tests: tab opens a new tab with 2 windows, vsplit stays in the current tab, inline opens a new tab with 1 window
• tests/backends/claudecode/test_edit.sh — new test_visible_only: verifies diff opens for a buffer visible in Neovim and is skipped for a file not open in any window
• tests/backends/claudecode/ (renamed from tests/backends/claude/) — all test scripts updated
• tests/helpers.sh — hook script paths updated to backends/claudecode/

Documentation

• README.md — fully updated: new module paths, command names, architecture diagram, testing commands, migration guide for users upgrading from older versions

Migration guide (for existing users)

1. Update require("claude-preview") → require("code-preview") in your Neovim config
2. Re-run :CodePreviewInstallClaudeCodeHooks (or OpenCode equivalent) to update hook script paths
3. Old :ClaudePreview* commands still work with a deprecation warning

Test plan

• All 39 tests pass (bash tests/run.sh) — 17 plugin + 22 backend
• tab / vsplit / inline layouts each produce the correct window structure
• visible_only skips diff for files not open in Neovim, shows diff for visible files
• Hook install writes new backends/claudecode/ paths to .claude/settings.local.json
• Hook uninstall removes both new and legacy markers (dual-marker transition)
• All old :ClaudePreview* commands work with deprecation warning
• Claude Code: edit / write / bash rm / sequential edits all work end-to-end
• OpenCode: edit / write / bash rm / multi-file queue / relative paths all work end-to-end
• visible_only, defer_claude_permissions, neo-tree reveal/indicators work on both backends
• Stale socket recovery works after Neovim restart

Issue:

#13