Add `gsd-tools state prune --keep-recent N` that moves old decisions
and recently-completed entries to STATE-ARCHIVE.md. Entries from phases
older than (current - N) are archived; the N most recent are kept.
STATE.md sections grow unboundedly in long-lived projects. A 20+ phase
project accumulates hundreds of historical decisions that every agent
loads into context. Pruning removes stale entries from the hot path
while preserving them in a recoverable archive.
Usage: gsd-tools state prune --keep-recent 3
Default: keeps 3 most recent phases
Closes#1970
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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