feat: structured RPC for hook scripts (#47, phase 2)

[Cannon07]

Phase 2 of #47 — replaces the string-interpolated "build Lua source in bash" pattern with structured JSON args dispatched through a single Lua entry point.

What changes

Before: Bash scripts built Lua source code as strings (\"require('...').show_diff('\$ESC_PATH', ...)\") and sent them via nvim --remote-expr. An escape_lua sed helper escaped every interpolated arg to avoid syntax errors when user data contained apostrophes, backslashes, or newlines. Easy to forget; one mistake = silent hook failure or quote breakout.

After: Bash builds a JSON args array with jq (which handles escaping correctly), writes it to a temp file, and calls nvim_call MOD FN JSON_ARGS. Inside nvim, code-preview.rpc.dispatch decodes the JSON and calls require(MOD)[FN](unpack(args)) with real Lua values. No user data ever enters a Lua source string — only the module name, function name, and temp-file path (all our own values) get interpolated.

Files

New

• `bin/nvim-call.sh` — `nvim_call(mod, fn, json_args)`. Returns 0/1/2 (ok / no socket / dispatch failed).
• `lua/code-preview/rpc.lua` — `dispatch(mod, fn, args_file)`. Errors wrap in pcall and route through `log.error` (vim.notify) so silent dispatch failures stay visible to the developer.

Removed

• `bin/nvim-send.sh` — `escape_lua` and the unused `nvim_send` function.

Migrated (13 callsites)

• `bin/core-pre-tool.sh` — `changes.set`, `neo_tree.refresh/reveal`, `hook_context`, `diff.show_diff` (Edit/Write/MultiEdit + ApplyPatch paths).
• `bin/core-post-tool.sh` — `diff.close_for_file`, `changes.clear_by_statuses`.
• `backends/{copilot,codex}/code-{preview,close}-diff.sh` — swapped six near-identical inline `vim.json.encode({debug=...})` queries for one `code-preview.log.state()` RPC.

Supporting Lua helpers

• `log.state()` — bundled `{debug, log_file, servername, cwd}` for the dedup above.
• `neo_tree.refresh_deferred(ms)` / `reveal_deferred(path, ms)` — so callers don't build `vim.defer_fn(function() ... end, ...)` as a string.
• `health.lua` — swapped `nvim-send.sh` for `nvim-call.sh`; added a load check for `code-preview.rpc`.

Pre-existing bug fixes uncovered during testing

Both are exactly the category of bash-quoting brittleness this migration was designed to retire — found them because Test 3 (filenames with apostrophes) finally exercised the unhappy paths:

1. rm path doubling. `detect_rm_paths` didn't strip surrounding quotes, so `rm "/abs/path"` failed the `[[ $p != /* ]]` test and got `$CWD/` prepended. Now strips outer ` ' ` / ` " ` and handles `~/` via a quoted case pattern.
2. xargs silently zeroing path lists. `RM_PATHS="$(echo "$RM_PATHS" | xargs)"` does shell-style quote processing — a single apostrophe in a filename (`it's-mine.txt`) made xargs see an unbalanced quote and return empty. Replaced with parameter-expansion trim that doesn't reinterpret the input.

Tested manually

• Edit / Write / MultiEdit / Create / Delete across Claude Code, Codex, OpenCode, Copilot CLI.
• Filenames with apostrophes (the case that surfaced both bug fixes above).
• `:checkhealth code-preview` passes clean, including the new dispatcher check.

Independent review

Asked a separate Claude Code session to audit the diff. Critical findings (silent dispatch failures, missing JSON-array validation, missing health check, undocumented field-shape contract) are all addressed in this PR. Reviewer's lower-priority findings (cosmetic / pre-existing / out-of-scope) deferred to follow-ups.

Out of scope

• Phase 3 (port `core-pre-tool.sh` → Lua, removes `nvim-call.sh` along the way).
• Phase 4 (same for `core-post-tool.sh`, removes `nvim-socket.sh`).

Test plan

• Edit/Write/MultiEdit across all four backends
• Bash-tool rm detection
• Filenames with apostrophes
• ApplyPatch (Codex)
• Accept/reject closes the diff
• :checkhealth code-preview

🤖 Generated with Claude Code