feat(forge): add gx keymap to open forge links in browser (#165)

Problem: Forge shorthand tokens like `gh:user/repo#42` are concealed in
the buffer so Neovim's built-in `gx` cannot recognize them as URLs.
Full URLs are also concealed, making `gx` unreliable.

Solution: Add `open_link` action (default `gx`) that reads `forge_spans`
from line metadata. If the cursor is on a forge token, that link opens;
otherwise the last ref on the line is used. Delegates to `vim.ui.open()`.

lua/pending/config.lua

@@ -83,6 +83,7 @@
 ---@field priority_down_visual? string|false
 ---@field cancelled? string|false
 ---@field edit_notes? string|false
+---@field open_link? string|false
 
 ---@class pending.CategoryViewConfig
 ---@field order? string[]
@@ -165,6 +166,7 @@ local defaults = {
     blocked = 'gb',
     cancelled = 'g/',
     edit_notes = 'ge',
+    open_link = 'gx',
     priority_up = '<C-a>',
     priority_down = '<C-x>',
     priority_up_visual = 'g<C-a>',