Use root repo hooks in linked worktrees

[abhinav-oai]

Why

Linked worktrees currently load their own project hook declarations, so the same repo can present different hook definitions depending on which checkout is active. #21762 tried to share trust by giving matching worktree hooks a shared synthetic key, but review pointed out that divergent worktree hook definitions would then fight over one trusted_hash.

Instead of introducing a second trust model, this makes linked worktrees use the root checkout as the single source of truth for project hook declarations. Worktree-local project config can still diverge for unrelated settings, but project hooks now keep one real source path and one trust state per repo.

What

• Teach project config loading to remember the matching root-checkout .codex/ folder for actual linked-worktree project layers.
• Keep ordinary project config sourced from the worktree, but replace project hook declarations with the root checkout's matching layer before hook discovery runs, including linked-worktree layers with .codex/ but no local config.toml.
• Make hook discovery use that authoritative hook folder for both hooks.json and TOML hook source paths, so linked worktrees produce the same hook key and trust state as the root checkout.
• Cover the linked-worktree path plus regressions for missing worktree config.toml and nested non-worktree project roots.