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/complete.lua

@@ -1,4 +1,5 @@
 local config = require('pending.config')
+local forge = require('pending.forge')
 
 ---@class pending.CompletionItem
 ---@field word string
@@ -109,6 +110,17 @@ local function recur_completions()
   return result
 end
 
+---@param source string
+---@return boolean
+function M._is_forge_source(source)
+  for _, b in ipairs(forge.backends()) do
+    if b.shorthand == source then
+      return true
+    end
+  end
+  return false
+end
+
 ---@type string?
 local _complete_source = nil
 
@@ -128,10 +140,10 @@ function M.omnifunc(findstart, base)
       { vim.pesc(dk) .. ':([%S]*)$', dk },
       { 'cat:([%S]*)$', 'cat' },
       { vim.pesc(rk) .. ':([%S]*)$', rk },
-      { 'gh:([%S]*)$', 'gh' },
-      { 'gl:([%S]*)$', 'gl' },
-      { 'cb:([%S]*)$', 'cb' },
     }
+    for _, b in ipairs(forge.backends()) do
+      table.insert(checks, { vim.pesc(b.shorthand) .. ':([%S]*)$', b.shorthand })
+    end
 
     for _, check in ipairs(checks) do
       local start = before:find(check[1])
@@ -172,7 +184,7 @@ function M.omnifunc(findstart, base)
         table.insert(matches, { word = c.word, menu = '[' .. source .. ']', info = c.info })
       end
     end
-  elseif source == 'gh' or source == 'gl' or source == 'cb' then
+  elseif M._is_forge_source(source) then
     local s = require('pending.buffer').store()
     if s then
       local seen = {}