Prioritize newer files in vault organizing skills

[markphelps]

What

Adds a ## Recency Priority section to the five vault skills that organize, compact, lint, and maintain the active surface: vault-lint, vault-compact, vault-concepts, vault-tracker, and the vault-maintain orchestrator.

Each skill now works the active surface newest-first, ranked by each object's updated frontmatter date (falling back to filesystem mtime when absent), and concentrates scrutiny on the freshest material — on the premise that long-untouched objects are already organized correctly.

Why

Older files have typically already been organized, so re-auditing them with equal weight wastes effort. Steering attention toward recently-touched objects — where active thinking lives and where new mismatches, overlaps, and gaps appear — makes each pass land where it matters most.

Design choices

• Prioritize, never exclude. Recency steers ordering and depth of scrutiny only. Old files are not skipped — they remain valid link targets, merge destinations, and canonical homes, and still get a structural check (broken links, orphaning).
• Signal: updated frontmatter, mtime fallback — aligns with the existing Portent model and vault-lint's stale-page check.
• Qualitative ordering only: no numeric day-count and no new flag, leaving judgment to scale to vault size.

Per-skill tailoring

Skill	Guidance
vault-lint	Order the issue list newest-first; a fresh file's issue outranks an older file's equivalent.
vault-compact	Seed overlap groups from recent files; the older established note stays the canonical merge destination.
vault-concepts	Weight theme detection toward recent notes, but keep temporal spread as the high-confidence signal.
vault-tracker	Surface recently-touched objects' mismatches first.
vault-maintain	Sub-audits inherit newest-first ordering; the weekly summary leads with recently-updated findings.

Source edits live in vault/skills/; the top-level skills/ copies are mirrored via scripts/sync-skills.sh (also run by the pre-commit hook). Plugin validator passes.