perf(ci): lean CI — testmon-select branches, shard postgres, drop depot, remove bossbot (#945)

Branch builds testmon-select against cached baselines; 3-shard Postgres on 3.14 only; free runners; 120s hang ceiling; non-code changes skip the matrix; BM Bossbot and infographic machinery removed.

Signed-off-by: phernandez <paul@basicmemory.com>

Author: phernandez