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>

spec/forge_spec.lua

@@ -391,6 +391,73 @@ describe('forge registry', function()
   end)
 end)
 
+describe('custom forge prefixes', function()
+  local config = require('pending.config')
+  local complete = require('pending.complete')
+
+  it('parses custom-length shorthand (3+ chars)', function()
+    local custom = forge.gitea_backend({
+      name = 'customforge',
+      shorthand = 'cgf',
+      default_host = 'custom.example.com',
+    })
+    forge.register(custom)
+
+    local ref = forge._parse_shorthand('cgf:alice/proj#99')
+    assert.is_not_nil(ref)
+    assert.equals('customforge', ref.forge)
+    assert.equals('alice', ref.owner)
+    assert.equals('proj', ref.repo)
+    assert.equals(99, ref.number)
+  end)
+
+  it('parse_ref dispatches custom-length shorthand', function()
+    local ref = forge.parse_ref('cgf:alice/proj#5')
+    assert.is_not_nil(ref)
+    assert.equals('customforge', ref.forge)
+    assert.equals(5, ref.number)
+  end)
+
+  it('find_refs finds custom-length shorthand', function()
+    local refs = forge.find_refs('Fix cgf:alice/proj#12')
+    assert.equals(1, #refs)
+    assert.equals('customforge', refs[1].ref.forge)
+    assert.equals(12, refs[1].ref.number)
+  end)
+
+  it('completion returns entries for custom backends', function()
+    assert.is_true(complete._is_forge_source('cgf'))
+  end)
+
+  it('config shorthand override re-registers backend', function()
+    vim.g.pending = {
+      forge = {
+        github = { shorthand = 'github' },
+      },
+    }
+    config.reset()
+    forge._reset_instances()
+
+    local ref = forge._parse_shorthand('github:user/repo#1')
+    assert.is_not_nil(ref)
+    assert.equals('github', ref.forge)
+    assert.equals('user', ref.owner)
+    assert.equals('repo', ref.repo)
+    assert.equals(1, ref.number)
+
+    assert.is_nil(forge._parse_shorthand('gh:user/repo#1'))
+
+    vim.g.pending = nil
+    config.reset()
+    for _, b in ipairs(forge.backends()) do
+      if b.name == 'github' then
+        b.shorthand = 'gh'
+      end
+    end
+    forge._reset_instances()
+  end)
+end)
+
 describe('forge diff integration', function()
   local store = require('pending.store')
   local diff = require('pending.diff')