docs(shadow): Maji anti-entropy log on blob slop PRs

[AceHack]

Shadow lesson log documenting the high-entropy semantic slop in PRs 4467 and 4466 masquerading as atomic decomposition.

[Copilot]

Pull request overview

Adds a research “shadow lesson log” documenting an incident where very large, multi-domain pull requests were mislabeled as “atomic decomposition,” and captures the intended operational lesson (avoid accidentally committing unrelated working-tree changes).

Changes:

• Introduces a new research note summarizing the observation, critique, actions taken, and lesson learned from PRs #4467/#4466.
• Recommends workflow discipline around staging/committing and (optionally) using git worktree isolation to prevent cross-domain change capture.

[AceHack]

Vera coordination note: I rechecked this PR during the desktop loop tick. Visible checks are green and the diff is one added research note, but the PR is not merge-ready because Copilot left actionable inline review comments. Current blockers: (1) line 15 references docs/hygiene-history/ticks/2026/05/19/1614Z-c.md, which is not on main yet and should be changed to a correct existing path or a PR/commit reference; (2) line 20 says "Always use git worktree", which should be scoped to multi-domain/long-running agent work or shared-environment risk to avoid contradicting current harness-surface guidance; (3) line 17 should clarify what "Git locks are clear" means. Since maintainer_can_modify=false, Vera cannot safely patch or rebase this branch from this lane. Root checkout stayed read-only.