docs(conventions): add the detector-findings owner doc (stub)

[kyle-sexton]

Phase 2 of the boris-routines-adoption plan. Depends on #2678.

A hard merge barrier. This must be on main before the Pattern-C pilot opens — a plugin cannot cite a repo-relative docs/conventions/ path, because it installs standalone. The repo's established form is a raw URL to main (plugins/architecture/reference/topic-docs.md:6, and four more), and plugins/skill-quality/scripts/check-skill.sh:480-495 fails broken refs.

Why a stub now rather than the full doc later

docs/PLUGIN-PHILOSOPHY.md:471 — "A new cross-plugin convention lands in an owner doc before a second plugin adopts it." That is a deadline, not a licence to author it late. The pilot phase makes plugins/mutation-testing the second plugin adopting the multi-producer rule, so a stub must precede it. The doc is then hardened from what the pilot observes, which is the real argument for depth trailing the pilot and it survives without :471.

Work items

• Create docs/conventions/detector-findings/README.md + CHANGELOG.md (the majority sibling shape — 12 of 20 convention directories carry a CHANGELOG).
• Own the multi-producer rule here, not in a review-plugin context file. PLUGIN-PHILOSOPHY.md:470 is "One owner doc per shared concern", and a rule binding three plugins cannot live inside one of them. feat(review): merge findings across producers and mark consumption explicitly (ADR 0010) #2678's file points here.
• Fix the four producer-owned fields: machine-computed Tier; Confidence high or omitted — never low, which ranks below absent (findings-normalization.md:62,72); repo-relative file:line Location; cell escaping (a literal pipe written backslash-pipe, newlines as spaces).
• Cite the findings-file schema by pointer to plugins/review/skills/fanout/context/default-mode.md. Never copy it.
• Add the row to the convention registry table at PLUGIN-PHILOSOPHY.md:474-499.

Sanity checks

• ls docs/conventions/detector-findings/README.md docs/conventions/detector-findings/CHANGELOG.md exits 0.
• grep -c "detector-findings" docs/PLUGIN-PHILOSOPHY.md >= 1.
• grep -c "^| Rank | Tier | Confidence" docs/conventions/detector-findings/README.md returns 0 — pointer-not-copy; the schema's owner is default-mode.md.
• gh pr view --json state -q .state on this PR returns MERGED before the pilot phase opens.

Related

• Depends on feat(review): merge findings across producers and mark consumption explicitly (ADR 0010) #2678
• Blocks the Pattern-C pilot and the hardening phase