feat(corrections): share the local findings buffer across a clone's worktrees #12907

Parent: #7555. Follows #12884 / #12889. The owner chose "whatever the research says" in session on 2026-11-03; the research recommends one buffer per clone, shared by every worktree.

Why

The local-file findings-buffer adapter keys its file on git rev-parse --show-toplevel (plugins/corrections/tools/findings-buffer/adapters/local-file/append.sh:17, read.sh:30), which is per worktree. A row noted in one worktree, including the applied-change rows /session-flow:retro now writes and judges, is invisible to a retro in any other worktree of the same clone. The owner runs many concurrent sessions across worktrees, so most later retros would find no rows.

• Claude Code's own auto memory uses the scope proposed here: "Auto memory is machine-local. All worktrees and subdirectories within the same git repository share one auto memory directory. Files are not shared across machines or cloud environments." (memory docs, auto memory).
• Rows stay on the machine: "Rows derived from session transcripts never leave the machine" (CONTRACT.md:87, Privacy). A GitHub-backed or committed store would loosen that control in a public repository; that is out of scope and the owner's call.
• git rev-parse --git-common-dir names the directory every linked worktree of a clone shares (git-rev-parse).

Done when

• local-file append.sh and read.sh resolve the buffer directory from git rev-parse --path-format=absolute --git-common-dir, so every worktree of one clone reads and writes one file, with the existing symlink and path guards applied to the new location.
• read.sh also returns rows from the old per-worktree <toplevel>/.work/buffer/findings.jsonl when present, so rows written before this change are not lost (buffer.sh read already drops repeat obs_ids).
• CONTRACT.md (and the corrections README where it names the path) states the new location and that it is shared by a clone's worktrees and stays on the machine.
• buffer.test.sh / conformance.test.sh gain a case: a row appended from a linked worktree is read from the main worktree, and the reverse.
• .changes/corrections/*.md fragment, bump: minor.
• CI green.

Out of scope

• Sharing rows across machines or through GitHub (Privacy rule above).