feat: git credential backend for credential storage (#371)

## Problem

Credentials were stored as plaintext JSON in
`stdpath('data')/cp-nvim.json`, with no integration with system
credential managers.

## Solution

Replace file-based credential storage with `git credential
fill/approve/reject`, delegating to whatever credential helper the user
has configured (`cache`, `store`, `libsecret`, macOS Keychain, etc.).

- New `lua/cp/git_credential.lua` module wrapping the git credential
protocol
- All credential consumers (`credentials.lua`, `submit.lua`,
`scraper.lua`) use `git_credential` directly — `cache.lua` no longer
handles credentials
- CSES API token packed into the password field (`password<TAB>token`)
so it works with helpers that ignore the `path` field
- `has_helper()` guard on `:CP login`, `:CP logout`, and `:CP submit`
with an error message if no helper is configured
- Healthcheck split into `[required]`/`[optional]` sections; git version
and credential helper status shown
- `git` checked at startup in `check_required_runtime()`
- Cache version system (`CACHE_VERSION`, v1→v2 migration) removed — the
cache file is now a plain JSON blob
- `:CP` command gets `bar = true`

lua/cp/utils.lua

@@ -366,6 +366,10 @@ function M.check_required_runtime()
     return false, timeout.reason
   end
 
+  if vim.fn.executable('git') ~= 1 then
+    return false, 'git is required for credential storage'
+  end
+
   return true
 end