bv v0.21.0

Features

defer_until is honored everywhere (#191): beads with a future
defer_until are now withheld from every ready surface — --recipe actionable, triage top picks, track/label picks, --robot-next
claimability (with an explicit "deferred until" reason), and the execution
plan. Past-dated and absent defer_until behave as before, and the field
loads on JSONL and both SQLite schema paths.

--robot-insights per-entry disk cache (v3 layout) (#192): the cached
path could be slower than --no-cache because cache validation walked the
entire .beads/ tree per lookup and the single-file cache decoded and
rewrote every entry of every repo on each hit. Validation now stats only
top-level .beads/ entries, and the cache is a per-entry layout with atomic
rename writes, lock-free single-entry reads, mtime-based pruning, and
corrupt/foreign-entry reaping. The legacy single-file cache is retired.

Robot mode no longer silently drops invalid issues (#190): issues
failing validation (e.g. updated_at < created_at) and JSONL parse drops
are surfaced in a load_stats block on every hand-rolled robot JSON
surface, so absence is distinguishable from "nothing to do". An
end-to-end test pins the inverted-timestamp case.

curl -fsSL https://raw.githubusercontent.com/Dicklesworthstone/beads_viewer/main/install.sh | bash
# or, from an existing install:
bv --update

Full Changelog: v0.20.0...v0.21.0