feat(build): add content freshness detection warnings

[s1ddh-rth]

Stop silently defaulting missing updated-on to today's date. Instead,
emit an actionable warning and set lastUpdated to null in the registry.

Add a staleness summary that counts entries not updated in 180+ days,
shown as an info line in human output and a stale field in JSON output.
Both validate-only and full build paths report consistently.

Closes #186

What

Two non-breaking additions to chub build:

1. Missing metadata.updated-on now emits a warning with a suggested fix instead of silently defaulting to today's date
2. A freshness summary counts entries older than 180 days and reports it in both human and JSON output

Why

Context Hub's value depends on content being current. Currently, missing updated-on is silently backfilled with today's date, masking the problem. There's also no visibility into how many entries in the registry are going stale. This change surfaces both issues as non-blocking warnings so contributors and maintainers can act on them.

Related: #106, #107

Testing

• npm test passes (231/231)
• chub build content/ --validate-only succeeds
• Manual testing done:
  • Verified missing updated-on emits actionable warning with suggested date
  • Verified --json output includes stale field
  • Verified freshness line only appears when stale entries exist (real content shows 0, test fixtures with old dates trigger correctly)
  • Verified no existing test assertions broken

Notes

• Only one file changed: cli/src/commands/build.js
• No new dependencies
• lastUpdated is set to null (not "unknown") when missing — cleaner for JSON consumers
• Staleness uses a Set to deduplicate by doc ID so multi-language/version docs aren't double-counted
• This is purely additive — no builds break, no new errors, just warnings and info