feat: v3.15.0 — page authoring + reach copy (closes #78, #79)

[vnykmshr]

Wave-4 of log.1mb.dev operator feedback closes the v3.13.0/v3.14.0 deferred threads for type:page first-class authoring (#79) and operator-voiced /about reach copy (#78). The four commits split as: new exported article.ValidateSlug contract for operator-supplied slugs (strict charset + length + reserved set {index,feed,rss,atom}); /compose/new-page route rendering compose.html in page mode via data-mode (slug field required, banner/tags/categories/link_url hidden, hidden type=page); three new ABOUT_REACH_HEADING / ABOUT_EMAIL_HEADING / ABOUT_EMAIL_INTRO env vars wired through the existing about_handler pattern with byte-exact defaults; and a regression test locking the type:page edit-roundtrip contract. Plan deviated at execution: the original Phase 1 (edit-loses-type fix) was dropped after TestUpdateArticle_PreservesAMAFields confirmed UpdateArticle already preserves unknown frontmatter via generic-map round-trip — plumbing absorbed into the page-affordance commit instead. Verified via make lint (cache-cleaned, 0 issues), make test green, an 11-scenario manual smoke matrix on the dev server, and byte-exact /about default rendering against the v3.14.0 baseline. Follow-up #80 filed for the adjacent BLOG_AUTHOR_EMAIL placeholder-default surface — deferred because it's a config-default change with its own scope.

Closes #78, #79.

[vnykmshr]

Code review

Found 1 issue:

1. ValidateSlug regex ^[a-z0-9-]+$ accepts slugs with leading or trailing hyphens (e.g. -mypage, mypage-) — but the pre-existing codebase-wide validSlug gate at internal/handlers/compose.go:22 (^[a-z0-9]([a-z0-9-]{0,198}[a-z0-9])?$) rejects them. A page submitted via /compose/new-page with slug -mypage passes validatePageSlug, the file is written to disk, then the operator's /compose/edit/-mypage request fails the validSlug.MatchString(slug) gate and returns 404. The page becomes uneditable via the compose UI (and unpublishable via PublishDraft, which uses the same gate). TestValidateSlug doesn't cover leading or trailing hyphens.

markgo/internal/services/article/predicate.go

Lines 55 to 60 in 1d9620e

	// slugCharClass enforces lowercase ASCII letters, digits, and hyphens.
	// generateSlug() in the compose service already produces this shape, so
	// auto-generated slugs are regex-compatible by construction.
	var slugCharClass = regexp.MustCompile(`^[a-z0-9-]+$`)
	// reservedSlugs blocks slugs that would confuse readers if served from

Suggested fix: change slugCharClass to mirror validSlug's anchored form (^[a-z0-9]([a-z0-9-]*[a-z0-9])?$) and add leading/trailing-hyphen rejection cases to TestValidateSlug.

markgo/internal/handlers/compose.go

Lines 20 to 24 in 1d9620e

	// validSlug matches URL-safe slugs: lowercase alphanumeric with hyphens, no leading/trailing hyphens, max 200 chars.
	var validSlug = regexp.MustCompile(`^[a-z0-9]([a-z0-9-]{0,198}[a-z0-9])?$`)
	const (