Parallelize independent worktree admission

[chubes4]

Summary

• atomically reserve bounded demand during global capacity admission
• release the global capacity lock before repository-scoped checkout
• merge live admission and bootstrap reservations without double-counting handles
• clean reservations on successful or failed allocation
• prove three independent repositories begin checkout within a bounded admission spread and overlap safely

Fixes #1308.

Verification

• php tests/worktree-parallel-multi-repo-admission.php twice
• php tests/workspace-capacity-lock-concurrency.php
• php tests/worktree-plan-capacity-hot-path.php
• php tests/worktree-add-lifecycle.php
• PHP syntax checks for modified source and new regression
• git diff --check

AI assistance

• Model: openai/gpt-5.6-sol
• Tool: OpenCode
• Used for: Investigated the global lock scope, implemented durable admission reservations, reviewed orphaned agent output, corrected the concurrency acceptance metric, and ran focused lifecycle verification.