feat: worktree extension apis

[sudo-tee]

This should address #513

Summary

Adds public APIs for worktree workflows without modifying internals or changing Neovim’s cwd. Addresses #513 and #511.

New APIs

Directory-bound sessions

require('opencode.api').open_session({
  directory = '/path/to/repo/.worktrees/feature',
  new = true,
  title = 'Feature work',
})

Opens a session in a logical panel tab bound to the requested directory.

Live custom slash commands

local opencode = require('opencode')

opencode.register_slash_command({
  name = 'worktree',
  desc = 'Open a worktree session',
  args = true,
  fn = function(args)
    return require('opencode.api').open_session({
      directory = args[1],
      new = true,
    })
  end,
})

-- Remove when no longer needed:
opencode.unregister_slash_command('worktree')

Per-directory session lock policies

lock_session_to_directory now accepts a function alongside the existing boolean:

require('opencode').setup({
  lock_session_to_directory = function(change)
    local function common_dir(dir)
      local result = vim.system({
        'git', '-C', dir, 'rev-parse',
        '--path-format=absolute', '--git-common-dir',
      }, { text = true }):wait()

      return result.code == 0 and vim.trim(result.stdout) or nil
    end

    local repo = common_dir(change.from)
    return repo ~= nil and repo == common_dir(change.to)
  end,
})

Behavior

Directory-bound tabs preserve their directory across tab switches. Completion and new sessions use that directory. Locked tabs retain their binding on cwd changes; unlocked tabs resume following cwd.

Includes documentation and tests for all three APIs.