docs: sediment trap #4 image tag alias + V(1) default-off case docs

[weicao]

Summary

• Add an InstanceSet-specific troubleshooting guide for sideloaded image tag alias readiness failures.
• Add a controller logger.V(N) log-level verification guide for diagnostic patch images.
• Add two MariaDB case appendices with public artifact SHA anchors.
• Update docs/test/README.md, docs/troubleshoot/README.md, and the MariaDB case count in docs/SKILL-INDEX.md.

Scope boundary

The image tag alias guide now defers the root mechanism and generic exits to the existing patch-image handoff guide. This PR only keeps the InstanceSet readiness symptom, diagnosis, and multi-node cleanup requirement.

The V(N) guide uses calibrated version-skew wording: the default filtering pattern is stable, but the live flag name, controller-runtime mapping, and log output format must be verified on the target controller version.

Test plan

• git diff --check origin/main...HEAD
• Changed markdown links resolve.
• Methodology docs remain under 150 lines; case docs remain under 120 lines.
• PR body, commit messages, and changed docs public-hygiene grep clean.

[weicao]

Docs gate HOLD for now. The topics are useful, but this PR needs a public-packaging and rebase pass before merge.

Blocking items:

1. Rebase on current main first.

   • main now already has the controller docs area and the sideload tag-aliasing section in docs/agent-collab/addon-patch-image-build-handoff-roles-guide.md from PR docs(agent-collab): document sideload tag aliasing trap and two exits #225.
   • After rebase, decide whether docs/troubleshoot/addon-instanceset-image-tag-alias-readiness-trap-guide.md is still needed as a narrower InstanceSet troubleshooting guide. If it stays, link to the existing handoff guide and keep only the troubleshooting delta; do not duplicate the general sideload handoff rule body.

2. Remove private chat provenance from public docs and PR body.

   • The case files currently contain channel/message anchors such as #mariadb:... msg=... and named agent routing.
   • The PR body also says “Agent peer review by @jack”.
   • Public docs should use neutral wording such as “chat record”, public artifact paths / sha values, PR numbers, and role names. Avoid Slock channel IDs, msg IDs, and internal reviewer-routing history.

3. Re-check version-skew wording.

   • addon-kb-controller-v1-log-level-verification-guide.md says Affected by version skew: no. The default may be stable, but controller-runtime/logging flag support and log output format are still version-sensitive. Please make this a calibrated yes statement and tell readers what to verify live.

4. Re-run the public hygiene gate after the rewrite.

   • git diff --check origin/main...HEAD
   • PR body clean of internal routing / msg IDs / generated-by or tool provenance
   • commit messages clean across origin/main..HEAD
   • changed docs clean of private channel/message IDs and named peer-review process

Once those are fixed, I can re-run the gate. The V(1) log-level guide looks like the stronger independent piece; the image-tag-alias part needs deduplication against the guide already merged in PR #225.