refactor: migrate credential consumers to git_credential

Problem: `credentials.lua`, `submit.lua`, and `scraper.lua` all
routed through `cache.get/set/clear_credentials`, which no longer
exists.

Solution: replace all `cache` credential calls with direct
`git_credential.get/store/reject` calls. Remove unused `cache`
imports where applicable.

lua/cp/credentials.lua

@@ -1,7 +1,7 @@
 local M = {}
 
-local cache = require('cp.cache')
 local constants = require('cp.constants')
+local git_credential = require('cp.git_credential')
 local logger = require('cp.log')
 local state = require('cp.state')
 
@@ -40,14 +40,14 @@ local function prompt_and_login(platform, display)
       end, function(result)
         vim.schedule(function()
           if result.success then
-            cache.set_credentials(platform, credentials)
+            git_credential.store(platform, credentials)
             logger.log(
               display .. ' login successful',
               { level = vim.log.levels.INFO, override = true }
             )
           else
             local err = result.error or 'unknown error'
-            cache.clear_credentials(platform)
+            git_credential.reject(platform, credentials)
             logger.log(
               display .. ' login failed: ' .. (constants.LOGIN_ERRORS[err] or err),
               { level = vim.log.levels.ERROR }
@@ -73,8 +73,7 @@ function M.login(platform)
 
   local display = constants.PLATFORM_DISPLAY_NAMES[platform] or platform
 
-  cache.load()
+  local existing = git_credential.get(platform) or {}
-  local existing = cache.get_credentials(platform) or {}
 
   if existing.username and existing.password then
     local scraper = require('cp.scraper')
@@ -91,7 +90,7 @@ function M.login(platform)
             { level = vim.log.levels.INFO, override = true }
           )
         else
-          cache.clear_credentials(platform)
+          git_credential.reject(platform, existing)
           prompt_and_login(platform, display)
         end
       end)
@@ -113,8 +112,10 @@ function M.logout(platform)
     return
   end
   local display = constants.PLATFORM_DISPLAY_NAMES[platform] or platform
-  cache.load()
+  local existing = git_credential.get(platform)
-  cache.clear_credentials(platform)
+  if existing then
+    git_credential.reject(platform, existing)
+  end
   local cookie_file = constants.COOKIE_FILE
   if vim.fn.filereadable(cookie_file) == 1 then
     local ok, data = pcall(vim.fn.json_decode, vim.fn.readfile(cookie_file, 'b'))

lua/cp/scraper.lua

@@ -347,7 +347,7 @@ 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)
+        require('cp.git_credential').store(platform, ev.credentials)
       end
       if ev.status ~= nil then
         if type(on_status) == 'function' then
@@ -395,7 +395,7 @@ 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)
+        require('cp.git_credential').store(platform, ev.credentials)
       end
       if ev.status ~= nil then
         if type(on_status) == 'function' then

lua/cp/submit.lua

@@ -1,8 +1,8 @@
 local M = {}
 
-local cache = require('cp.cache')
 local config = require('cp.config')
 local constants = require('cp.constants')
+local git_credential = require('cp.git_credential')
 local logger = require('cp.log')
 local state = require('cp.state')
 
@@ -14,7 +14,7 @@ local STATUS_MSGS = {
 }
 
 local function prompt_credentials(platform, callback)
-  local saved = cache.get_credentials(platform)
+  local saved = git_credential.get(platform)
   if saved and saved.username and saved.password then
     callback(saved)
     return
@@ -109,12 +109,12 @@ function M.submit(opts)
       function(result)
         vim.schedule(function()
           if result and result.success then
-            cache.set_credentials(platform, creds)
+            git_credential.store(platform, creds)
             logger.log('Submitted successfully', { level = vim.log.levels.INFO, override = true })
           else
             local err = result and result.error or 'unknown error'
             if err == 'bad_credentials' or err:match('^Login failed') then
-              cache.clear_credentials(platform)
+              git_credential.reject(platform, creds)
               logger.log(
                 'Submit failed: ' .. (constants.LOGIN_ERRORS[err] or err),
                 { level = vim.log.levels.ERROR }