feat: Add .gtrconfig file for declarative configuration

[K9i-0]

Summary

Add support for a .gtrconfig file that allows all gtr settings to be declared in a single file and committed to the repository.

Motivation

Currently, most settings require running git gtr config commands manually. This creates friction for team usage - each developer must set up hooks, excludes, etc. individually.

While .worktreeinclude (#28) allows sharing copy patterns via the repository, other settings still require manual setup:

Setting	Config commands	File-based
gtr.copy.include	✅	✅ .worktreeinclude
gtr.copy.exclude	✅	❌
gtr.copy.includeDirs / excludeDirs	✅	❌
gtr.hook.postCreate / postRemove	✅	❌
gtr.editor.default / gtr.ai.default	✅	❌

Proposal

A .gtrconfig file that covers all settings:

copy:
  include:
    - "**/.env.example"
    - "*.md"
  exclude:
    - "**/.env"
  includeDirs:
    - node_modules
  excludeDirs:
    - node_modules/.cache

hooks:
  postCreate:
    - "pnpm install"

defaults:
  editor: cursor
  ai: claude

This allows teams to commit configuration and share it across developers.