improvement: embed corpus_date build timestamp via ldflags

[castrojo]

Finding

Users who built their binary 3 months ago have stale embedded discussion corpus.
The binary has no way to tell the AI client how old its seed data is.

Fix

Add to cmd/bluefin-mcp/main.go:

var buildDate = "unknown"  // -ldflags "-X main.buildDate=2025-03-20"

Include in all search_discussions responses:

{"results": [...], "corpus_date": "...", "build_date": "..."}

Also update Justfile to pass -ldflags at build time.

Labels: enhancement, priority:low