chore: align .markdownlint.json with the canonical fleet ruleset

[twistedmelonman]

Standards Check has never run on this repo -- the workflow is active but
no PR has been opened since it landed, so its first run would have
reported 31 markdownlint findings.

All 31 are MD040 (fenced code without a language) and MD060, both of
which are disabled fleet-wide in smartwatermelon/github-workflows
standards/markdownlint.json. They appear only because this repo carries
its own .markdownlint.json, and run-standards.sh:130 prefers a repo-local
config over the canonical one -- it replaces rather than merges, so the
repo was being linted against a stricter, ad-hoc ruleset that no other
repo uses.

The local config was not a deliberate choice to be stricter. It arrived
incidentally in #21 (2026-04-07), a CLAUDE.md documentation commit that
predates the fleet standards work, with no rationale recorded for its
rules and nothing in the repo referencing it.

Replacing it with the canonical file byte-for-byte takes the count to
zero without editing a single document. The alternative -- adding
language tags to 21 code fences to satisfy a rule the fleet has turned
off -- would be churn in service of a local accident.

Verification:

before (repo-local config): 31 findings
after (canonical config): 0 findings, exit 0
sha256 matches canonical: d1cf00627c596386

Known-bad control: an MD012 injected into README.md is still reported
(2 hits), so the file still lints -- this is an alignment, not a
silencing.

Advances smartwatermelon/dev-env#110

https://claude.ai/code/session_01TkReZXv8XkcWNbcaiWfcvg

[claude]

Review Summary

Scope: .markdownlint.json — markdown linting configuration

Changes: Addition of 5 new linting rules with appropriate configurations:

• MD024 (duplicate headings): siblings_only restriction
• MD029 (ordered list prefix): disabled
• MD040 (fenced code block language): disabled
• MD046 (code block style): disabled
• MD060 (table pipe table): disabled

Assessment: JSON syntax is valid. No production code changes. Linting configuration updates have no impact on runtime behavior, reliability, or security.

VERDICT: PASS