## 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`
## Problem
After the initial CodeChef implementation, submit silently swallowed
"contest not available for submission" errors, and the submit flow used
blind `wait_for_timeout` delays. AtCoder had an unnecessary 500ms settle
delay after file upload.
## Solution
Replace the initial page-load sleep in CodeChef submit with
`wait_for_selector`, replace the 3s post-click sleep with a proper
`wait_for_selector` on the result dialog, and extend the practice
fallback
check to catch both dialog variants. Remove AtCoder's
`BROWSER_SETTLE_DELAY`
and the constant from `timeouts.py`.
Problem: time and timeout were listed as optional dependencies despite
being required for plugin initialization. nix was not mentioned as an
alternative to uv for the Python scraping environment.
Solution: rename section to "Dependencies", list time/timeout first,
and add nix as an alternative to uv for scraping.
