feat(gc): add pkgmgr gc --report, a read-only disk inventory

pkgmgr gc --report lists each linked worktree and orphaned .claude/worktrees/agent-* dir with its size and last activity, the changes a cleanup would keep, the biggest children of every --root, ELF core dumps, docker system df with --docker, and free versus attributed space per filesystem; --json prints the same as JSON. It deletes and writes nothing and refuses --apply. git run() gains input_text for cat-file --batch-check.

🎯 Fix: 90 %
💥 Break: 2 %
💸 Cost: new
⏱️ Time: new
🛡️ Security: 100 %
🏆 Score: 88
🚦 Gate: cost new, time new: cleared by the operator's commit word for pkgmgr gc --report (2026-11-07)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -79,6 +79,7 @@ versioning features it can drive **end-to-end workflows**:
 | `pkgmgr branch cleanup [--apply]` | Removes merged branches that no worktree holds, locally and on the remotes |
 | `pkgmgr branch tree [--format json\|yaml] [--output FILE]` | Writes every managed repository's branches and worktrees with their cleanup state, without fetching |
 | `pkgmgr gc [--all] [--apply]` | Both cleanups in one pass, for the current repository or every managed one |
+| `pkgmgr gc --report [--all] [--root DIR] [--threshold-mb MB] [--docker] [--json]` | Read-only disk inventory: worktree sizes, changes a cleanup would keep, `--root` children, core dumps, docker usage, free space per filesystem |
 | `pkgmgr worktree orphans --stale-days N --top N` | Lists merged and abandoned branches and worktrees with a proposal each |
 
 A hook runs only when the `worktree_hooks` config pins its command under the