[codex] Add WP object edit-delete merge smoke coverage

[adamziel]

What it does

Adds fast COW merge smoke regressions for two WordPress object graphs where the source branch edits an object while the target branch deletes it:

• wp_comments plus wp_commentmeta
• a CPT-like forkpress_note graph across wp_posts, wp_postmeta, wp_options, and wp_term_relationships

It also updates the merge reliability matrix to record the v0.1.28 release gate and the new edit/delete coverage.

Rationale

The smoke suite already covered create/create merges and edit/delete conflicts for reusable blocks, menus, and attachments. Comments and CPT rows are also complete WordPress object graphs. These cases should stay reviewable and preserve the target deletion before review instead of silently resurrecting target-deleted content.

Implementation

The new fixtures assert:

• completed_with_conflicts for source-edit/target-delete graph races
• target deletion is preserved before review
• each changed source row records a row-target-deleted conflict
• default decisions are target-wins until a reviewer resolves the conflict

Testing instructions

php -l tests/cow/merge_smoke.php
git diff --check
make test-cow-merge-smoke