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/scraper.lua

@@ -347,7 +347,9 @@ function M.login(platform, credentials, on_status, callback)
     stdin = vim.json.encode(credentials),
     on_event = function(ev)
       if ev.credentials ~= nil and next(ev.credentials) ~= nil then
-        require('cp.cache').set_credentials(platform, ev.credentials)
+        vim.schedule(function()
+          require('cp.git_credential').store(platform, ev.credentials)
+        end)
       end
       if ev.status ~= nil then
         if type(on_status) == 'function' then
@@ -395,7 +397,9 @@ function M.submit(
     stdin = vim.json.encode(credentials),
     on_event = function(ev)
       if ev.credentials ~= nil and next(ev.credentials) ~= nil then
-        require('cp.cache').set_credentials(platform, ev.credentials)
+        vim.schedule(function()
+          require('cp.git_credential').store(platform, ev.credentials)
+        end)
       end
       if ev.status ~= nil then
         if type(on_status) == 'function' then