feat: add g:dpp#hook_result for hook return values

[Copilot]

Hooks are executed via execute() which cannot return values, making it impossible for hook code to communicate results back to the caller — particularly when hooks are defined as plain strings in TOML.

Introduces a convention using g:dpp#hook_result: the variable is reset to v:null before each hook execution, and the hook can assign any value to it.

Changes

• autoload/dpp/util.vim: Reset g:dpp#hook_result = v:null at the top of dpp#util#_execute_hook() before running hook code
• doc/dpp.txt: New VARIABLES section documenting g:dpp#hook_result; usage note added to the HOOKS section

Usage (TOML-compatible)

hook_source = '''
  let g:dpp#hook_result = s:my_setup()
'''

Read g:dpp#hook_result immediately after the hook of interest — it is overwritten on each subsequent hook execution.