feat(cli): native date-range filtering for search commands

[Flare576]

Context

Discovered while building the ei-generate skill's period-scoped document types (Profile/Job Description's session-confidence facet; a new Period Performance Review type). There is no native date-range filter on any search command today.

The current workaround, verified live against production data on 2026-08-03:

• ei quotes --recent -n 100000 returns the full quote corpus unfiltered by the similarity floor (12,464 quotes spanning Sept 2025 -> Aug 2026), and each flat result already carries timestamp -- so date-range filtering is possible client-side today, just wasteful (pulling everything to filter a slice).
• ei facts --recent -n <large> behaves the same way; flat results carry validated_date, but that's "when validated," not "when mentioned" -- not useful for a period query on its own.
• Topics and People are the real gap. last_mentioned/last_updated exist on the underlying type (src/core/types/data-items.ts:14, "Used for --recent sorting") and are used internally to order --recent results, but the CLI command mappers (src/cli/commands/topics.ts, people.ts) strip both fields from the flat search result before returning it. ei --id <id> on the full record does expose them -- but doing that per-candidate across thousands of topics to find "what falls in May" is impractical. The workaround: pull date-bounded quotes (real dates) and roll up their linked_items to infer topic/person involvement for the period, rather than iterating --id over every topic.

Proposal

1. Add --since <date> / --until <date> (or --after/--before) flags to the balanced search and all typed search commands (ei, ei quotes, ei topics, ei facts, ei people), filtering on the relevant date field (timestamp for quotes, last_mentioned/last_updated for topics/facts/people) before the -n truncation -- not as a client-side afterthought requiring an inflated -n.
2. Expose last_mentioned/last_updated in the flat topic/fact/person search result shape (not just on ei --id full-record lookup), so date-based reasoning doesn't require an extra round trip per candidate.
3. Document the new flags in ei --help.
4. Preserve existing --recent (sort-by-recency, no range) behavior for callers that don't specify a range.

Acceptance Criteria

• ei <type> --since 2026-05-01 --until 2026-06-01 "query" (and the balanced ei --since ... --until ... form) filters results to the given range without requiring -n inflation as a workaround.
• Flat search results for topics, facts, and people include their date field so period-based work doesn't require a follow-up ei --id call per candidate.
• ei --help documents --since/--until.
• skills/ei-generate/references/recon.md and any other skill doc currently instructing the -n-inflation workaround (the RoboBrain session-confidence facet, the Period Performance Review facet, once shipped) are updated to use the native flags instead.
• Existing --recent behavior with no range specified is unchanged.