feat(forge): support custom shorthand prefixes

Problem: forge shorthand parsing hardcoded `%l%l` (exactly 2 lowercase
letters), preventing custom prefixes like `github:`. Completions also
hardcoded `gh:`, `gl:`, `cb:` patterns.

Solution: iterate `_by_shorthand` keys dynamically in `_parse_shorthand`
instead of matching a fixed pattern. Build completion patterns from
`forge.backends()`. Add `shorthand` field to `ForgeInstanceConfig` so
users can override prefixes via config, applied in `_ensure_instances()`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lua/pending/forge.lua

@@ -62,6 +62,14 @@ function M.backends()
   return _backends
 end
 
+function M._reset_instances()
+  _instances_resolved = false
+  _by_shorthand = {}
+  for _, b in ipairs(_backends) do
+    _by_shorthand[b.shorthand] = b
+  end
+end
+
 local function _ensure_instances()
   if _instances_resolved then
     return
@@ -73,17 +81,27 @@ local function _ensure_instances()
     for _, inst in ipairs(forge_cfg.instances or {}) do
       _by_host[inst] = backend
     end
+    if forge_cfg.shorthand and forge_cfg.shorthand ~= backend.shorthand then
+      _by_shorthand[backend.shorthand] = nil
+      backend.shorthand = forge_cfg.shorthand
+      _by_shorthand[backend.shorthand] = backend
+    end
   end
 end
 
 ---@param token string
 ---@return pending.ForgeRef?
 function M._parse_shorthand(token)
-  local prefix, rest = token:match('^(%l%l):(.+)$')
-  if not prefix then
-    return nil
+  _ensure_instances()
+  local backend, rest
+  for prefix, b in pairs(_by_shorthand) do
+    local candidate = token:match('^' .. vim.pesc(prefix) .. ':(.+)$')
+    if candidate then
+      backend = b
+      rest = candidate
+      break
+    end
   end
-  local backend = _by_shorthand[prefix]
   if not backend then
     return nil
   end