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/init.lua

@@ -0,0 +1,80 @@
+local M = {}
+
+---@alias forge.Segment {[1]: string, [2]: string?}
+
+---@class forge.PickerEntry
+---@field display forge.Segment[]
+---@field value any
+---@field ordinal string?
+
+---@class forge.PickerActionDef
+---@field name string
+---@field fn fun(entry: forge.PickerEntry?)
+
+---@class forge.PickerOpts
+---@field prompt string?
+---@field entries forge.PickerEntry[]
+---@field actions forge.PickerActionDef[]
+---@field picker_name string
+
+---@type table<string, string>
+local backends = {
+  ['fzf-lua'] = 'forge.picker.fzf',
+  telescope = 'forge.picker.telescope',
+  snacks = 'forge.picker.snacks',
+}
+
+---@return string
+local function detect()
+  local cfg = require('forge').config()
+  local name = cfg.picker or 'auto'
+  if name ~= 'auto' then
+    return name
+  end
+  if pcall(require, 'fzf-lua') then
+    return 'fzf-lua'
+  end
+  if pcall(require, 'snacks') then
+    return 'snacks'
+  end
+  if pcall(require, 'telescope') then
+    return 'telescope'
+  end
+  return 'fzf-lua'
+end
+
+---@param entry forge.PickerEntry
+---@return string
+function M.ordinal(entry)
+  if entry.ordinal then
+    return entry.ordinal
+  end
+  local parts = {}
+  for _, seg in ipairs(entry.display) do
+    table.insert(parts, seg[1])
+  end
+  return table.concat(parts)
+end
+
+---@return string
+function M.backend()
+  return detect()
+end
+
+---@param opts forge.PickerOpts
+function M.pick(opts)
+  local name = detect()
+  local mod_path = backends[name]
+  if not mod_path then
+    vim.notify('[forge]: unknown picker backend: ' .. name, vim.log.levels.ERROR)
+    return
+  end
+  local ok, backend = pcall(require, mod_path)
+  if not ok then
+    vim.notify('[forge]: picker backend ' .. name .. ' not available', vim.log.levels.ERROR)
+    return
+  end
+  backend.pick(opts)
+end
+
+return M