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/fzf.lua

@@ -0,0 +1,76 @@
+local M = {}
+
+local fzf_args = (vim.env.FZF_DEFAULT_OPTS or '')
+  :gsub('%-%-bind=[^%s]+', '')
+  :gsub('%-%-color=[^%s]+', '')
+
+---@param key string
+---@return string
+local function to_fzf_key(key)
+  if key == '<cr>' then
+    return 'default'
+  end
+  local result = key:gsub('<c%-(%a)>', function(ch)
+    return 'ctrl-' .. ch:lower()
+  end)
+  return result
+end
+
+---@param segments forge.Segment[]
+---@return string
+local function render(segments)
+  local utils = require('fzf-lua.utils')
+  local parts = {}
+  for _, seg in ipairs(segments) do
+    if seg[2] then
+      table.insert(parts, utils.ansi_from_hl(seg[2], seg[1]))
+    else
+      table.insert(parts, seg[1])
+    end
+  end
+  return table.concat(parts)
+end
+
+---@param opts forge.PickerOpts
+function M.pick(opts)
+  local cfg = require('forge').config()
+  local keys = cfg.keys
+  if keys == false then
+    keys = {}
+  end
+  local bindings = keys[opts.picker_name] or {}
+
+  local lines = {}
+  for i, entry in ipairs(opts.entries) do
+    lines[i] = ('%d\t%s'):format(i, render(entry.display))
+  end
+
+  local fzf_actions = {}
+  for _, def in ipairs(opts.actions) do
+    local key = def.name == 'default' and '<cr>' or bindings[def.name]
+    if key then
+      fzf_actions[to_fzf_key(key)] = function(selected)
+        if not selected[1] then
+          def.fn(nil)
+          return
+        end
+        local idx = tonumber(selected[1]:match('^(%d+)'))
+        def.fn(idx and opts.entries[idx] or nil)
+      end
+    end
+  end
+
+  require('fzf-lua').fzf_exec(lines, {
+    fzf_args = fzf_args,
+    prompt = opts.prompt or '',
+    fzf_opts = {
+      ['--ansi'] = '',
+      ['--no-multi'] = '',
+      ['--with-nth'] = '2..',
+      ['--delimiter'] = '\t',
+    },
+    actions = fzf_actions,
+  })
+end
+
+return M