Add time-based filters to search and listing APIs

[sebkouba]

Problem

QMD had no first-class way to constrain search or listing to recently created or modified documents. Users and MCP/SDK callers had to encode recency indirectly in query text, and qmd ls could not filter by indexed timestamps at all.

Solution

• add --modified-since / --created-since to qmd search, qmd query, qmd vsearch, and qmd ls
• thread modifiedAfter / createdAfter through the store, SDK, and MCP search APIs
• apply filters in SQL using the indexed documents.modified_at and documents.created_at metadata
• document the feature in the README and QMD skill
• sync the packaged embedded skill with skills/qmd/SKILL.md and harden CLI tests so packaged skill drift is caught

Notes

Time filters operate on indexed filesystem timestamps, so callers still need qmd update when they want the latest metadata reflected.

Testing

• bunx vitest run --reporter=verbose test/store.helpers.unit.test.ts test/cli.test.ts test/sdk.test.ts test/mcp.test.ts
• CLI coverage for search, structured query, listing, and invalid values
• SDK coverage for modified/created filters, inclusive boundaries, combined filters, and vector search
• MCP coverage for the new search tool parameters