refactor(forge): replace curl/token auth with CLI-native API calls

Problem: Forge metadata fetching required manual token management —
config fields, CLI token extraction, and curl with auth headers. Each
forge had a different auth path, and Codeberg had no CLI support at all.

Solution: Delete `get_token()` and `_api_url()`, replace with
`_api_args()` that builds `gh api`, `glab api`, or `tea api` arg
arrays. The CLIs handle auth internally. Add `warn_missing_cli` config
(default true) that warns once per forge per session on failure. Add
forge CLI checks to `:checkhealth`. Remove `token` from config/docs.

lua/pending/config.lua

@@ -34,13 +34,13 @@
 ---@field region? string
 
 ---@class pending.ForgeInstanceConfig
----@field token? string
 ---@field icon? string
 ---@field issue_format? string
 ---@field instances? string[]
 
 ---@class pending.ForgeConfig
 ---@field auto_close? boolean
+---@field warn_missing_cli? boolean
 ---@field github? pending.ForgeInstanceConfig
 ---@field gitlab? pending.ForgeInstanceConfig
 ---@field codeberg? pending.ForgeInstanceConfig
@@ -156,6 +156,7 @@ local defaults = {
   sync = {},
   forge = {
     auto_close = false,
+    warn_missing_cli = true,
     github = {
       icon = '',
       issue_format = '%i %o/%r#%n',