feat(picker): add multi-backend picker abstraction

Problem: all pickers were tightly coupled to fzf-lua via ANSI strings
and fzf-specific action tables, making it impossible to use telescope
or snacks.nvim.

Solution: introduce `forge.picker` dispatcher with `fzf`, `telescope`,
and `snacks` backends. Format functions now return `forge.Segment[]`
instead of ANSI strings. `pickers.lua` builds backend-agnostic
`forge.PickerEntry[]` and delegates to `forge.picker.pick()`. Backend
auto-detection tries fzf-lua, snacks, telescope in order. Commits,
branches, and worktree pickers remain fzf-only with graceful fallback.

lua/forge/picker/snacks.lua

@@ -0,0 +1,64 @@
+local M = {}
+
+---@param opts forge.PickerOpts
+function M.pick(opts)
+  local Snacks = require('snacks')
+  local picker_mod = require('forge.picker')
+
+  local cfg = require('forge').config()
+  local keys = cfg.keys
+  if keys == false then
+    keys = {}
+  end
+  local bindings = keys[opts.picker_name] or {}
+
+  local items = {}
+  for i, entry in ipairs(opts.entries) do
+    items[i] = {
+      idx = i,
+      text = picker_mod.ordinal(entry),
+      value = entry,
+    }
+  end
+
+  local snacks_actions = {}
+  local input_keys = {}
+  local list_keys = {}
+  for _, def in ipairs(opts.actions) do
+    local key = def.name == 'default' and '<cr>' or bindings[def.name]
+    if key then
+      local action_name = 'forge_' .. def.name
+      snacks_actions[action_name] = function(picker)
+        local item = picker:current()
+        picker:close()
+        def.fn(item and item.value or nil)
+      end
+      if key == '<cr>' then
+        snacks_actions['confirm'] = snacks_actions[action_name]
+      else
+        -- selene: allow(mixed_table)
+        input_keys[key] = { action_name, mode = { 'i', 'n' } }
+        list_keys[key] = action_name
+      end
+    end
+  end
+
+  Snacks.picker({
+    items = items,
+    prompt = opts.prompt,
+    format = function(item)
+      local ret = {}
+      for _, seg in ipairs(item.value.display) do
+        table.insert(ret, { seg[1], seg[2] or 'Normal' })
+      end
+      return ret
+    end,
+    actions = snacks_actions,
+    win = {
+      input = { keys = input_keys },
+      list = { keys = list_keys },
+    },
+  })
+end
+
+return M