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/config.lua

@@ -37,6 +37,7 @@
 ---@field icon? string
 ---@field issue_format? string
 ---@field instances? string[]
+---@field shorthand? string
 
 ---@class pending.ForgeConfig
 ---@field auto_close? boolean