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`

doc/cp.nvim.txt

@@ -49,7 +49,8 @@ REQUIREMENTS                                                 *cp-requirements*
 
 - Neovim 0.10.0+
 - Unix-like operating system
-- uv package manager (https://docs.astral.sh/uv/)
+- (Optional) git 1.7.9+ (credential storage)
+- (Optional) uv package manager (https://docs.astral.sh/uv/)
 
 ==============================================================================
 SETUP                                                              *cp-setup*
@@ -998,8 +999,15 @@ CREDENTIALS                                                   *cp-credentials*
 
 Manage stored login credentials for platform submission.
 
-Credentials are stored under _credentials in the main cache file
-(stdpath('data')/cp-nvim.json). Use :CP cache read to inspect them.
+Credentials are stored via git-credential(1), using whatever credential
+helper is configured in your git config (macOS Keychain, libsecret,
+credential-store, etc.). Git is required. Cookie files
+(~/.cache/cp-nvim/cookies.json) are unaffected.
+
+To inspect stored credentials:
+>sh
+    printf 'protocol=https\nhost=cses.fi\n\n' | git credential fill
+<
 
 :CP login [platform]
         Set or update credentials for a platform. Prompts for username