chore: enforce 7 more markdownlint rules, fix pre-existing violations

[Arthurvdv]

Follow-up to #171. Resolves the markdownlint debt that shipped disabled to get the gate green, and re-evaluates the "deliberate convention" disables. Stacked on ci/pr-build (base will move to main once #171 merges).

Result: 20 disabled → 13 disabled, 7 now enforced

Enforced now (were disabled):

• MD009, MD012, MD031, MD032, MD047 — whitespace hygiene, cleared via markdownlint --fix across ~20 files (trailing spaces, blank-line normalization, final newline). All changes verified prose-safe.
• MD055, MD056 — table pipe style / column count. The only offenders repo-wide were two malformed borderless tables in LinterCop/LC0090; reformatted to bordered pipes with correct column counts (data preserved, renders as 3 correct tables).

The key finding

115 content files author code with Hugo shortcodes ({{< highlight >}}, {{< tab >}}, {{< tabpane >}}), which markdownlint parses as prose. That makes a whole class of rules structural false positives, not fixable debt — there's no config to make markdownlint read shortcode bodies. Verified with the exact CI tool (markdownlint-cli2@0.23.2).

A concrete example caught during implementation: --fix with MD038 enabled corrupted a PowerShell backtick line-continuation in github.md (stripped indentation, reading the backticks as a code span). MD038 was reverted and moved to the disabled set.

Stays disabled — now categorized with reasons in .markdownlint.jsonc

• Structural false positive (code in shortcodes): MD033, MD018, MD025, MD026, MD034, MD038.
• Deliberate convention: MD013 (prose authored as long unwrapped lines), MD001 (rule pages open at ### Example under the front-matter title; cli2 counts the title as H1 → h1→h3).
• Deferred manual debt (future pass): MD022, MD040, MD046, MD036, MD060.

Docs sync

.claude/rules/rule-pages.md now documents bordered pipe tables with consistent column counts (supports MD055/MD056). The /new-rule skill already emits compliant tables and clean Markdown — no change needed.

Verification (local, current tree)

• markdownlint-cli2@0.23.2 "content/**/*.md" → 0 issues, 148 files.
• hugo --gc --minify --panicOnWarning --printPathWarnings → clean.
• htmltest → 157 docs pass (LC0090 tables render; #compensating-usages anchor intact).

Out of scope (noted for later)

• Migrating shortcode code blocks → fenced blocks with {hl_lines} would unlock MD018/MD025/MD026/MD033/MD034 but touches 115 files, rule-pages.md, and /new-rule — a separate initiative.
• Clearing the deferred-debt rules and the pre-existing broken links (still allowlisted in .htmltest.yml) remain their own follow-ups.

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