fix(docs): frontmatter disclosure, sidenav directory tree, themed syntax highlighting

[cameronsjo]

Three rendering defects in forgectl docs serve, all surfaced by looking at the reader on real docs:

• Frontmatter mangling — goldmark read the opening --- as a thematic break and folded the YAML into a broken heading. The frontmatter extension now consumes it at parse; the parsed pairs render as a collapsed "Front matter · N keys" disclosure (kv grid) above the body. The block is built after sanitization from html.EscapeString'd fragments only, so the bluemonday allowlist stays untouched. A well-formedness gate protects docs that merely open with a thematic break — the extension's greedy opener otherwise swallowed the whole body (found by review, pinned by test).
• Flat sidenav — 800+ docs rendered as one flat scroll while the vendored tree behavior sat unused. Each root now renders as a nested tree--static (native details/summary, no JS needed): per-directory counts, path to the current doc pre-expanded, filter box hides empty branches and auto-expands matches.
• Monokai code blocks — the served chroma stylesheet was a generated monokai sheet with a hardcoded palette. Replaced with a hand-authored mapping onto the design system's syntax roles; code blocks now follow the light/dark theme.

Reviewed pre-PR: two independent code reviews (findings folded, incl. the body-swallowing gate), an independent Opus security review (verdict clean — the post-sanitizer prepend was empirically probed with hostile YAML: anchors, block scalars, angle-bracketed keys; zero OSV advisories on the two new deps), scoped lint clean, go build ./... && go test ./... green.

BEGIN_COMMIT_OVERRIDE
fix(docs): the reader now renders YAML/TOML frontmatter as a collapsed metadata disclosure instead of leaking it into the body as a broken heading; each indexed root renders as a collapsible directory tree (counts, current-path pre-expanded, filter-aware) instead of a flat list; and syntax highlighting follows the light/dark theme instead of a fixed monokai palette
END_COMMIT_OVERRIDE

Session-Name: deft-sonata
Session-Id: 7fc5913c-2346-479f-a249-9d871812e47d
Model: claude-fable-5
Harness: claude-code 2.1.252
Machine: cf6e768835c7

[coderabbitai]

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/docs-frontmatter-and-tree

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