feat(session): add lifecycle management — storage reclamation, CLI commands, VACUUM support

[guazi04]

Summary

Implements Session Lifecycle Management (MVP path from #16101). Sessions now have a lifecycle — not just a birth.

What's new

Post-compaction storage reclamation (compaction.ts)

• prune() now clears output and attachments from ToolStateCompleted parts after marking time.compacted — previously it only marked but never reclaimed
• Added retroactive reclamation pass in process() that catches previously-pruned parts with unreclamed content
• Gated by new compaction.reclaim config option (default: true)

CLI session management commands (cli/cmd/session.ts)

• opencode session archive <id> / unarchive <id> — surfaces existing setArchived() functionality that was API-only
• opencode session stats — shows DB/WAL file size, session counts (root/child/archived), message/part counts via efficient SQL aggregates
• opencode session prune — bulk cleanup with --older-than (default 30 days), --children, --vacuum, --dry-run

VACUUM support (storage/db.ts)

• Database.vacuum() — WAL checkpoint (TRUNCATE) + VACUUM to actually free disk space
• Database.checkpoint() — WAL checkpoint only

Data integrity fix (session/index.ts)

• Added childrenAll() that queries children by parent_id only (no project filter)
• remove() now uses childrenAll() for cascade deletion, fixing cross-project child session orphaning

Design decisions

• Conservative reclamation: Only clears parts already marked as compacted — doesn't touch active/recent content
• Direct Drizzle queries for prune: Avoids listGlobal() ordering bias that would skip the oldest sessions
• Descendant tracking: Prune uses Set<string> to prevent double-delete when --children includes both parent and child
• Vacuum safety: Wrapped in try/catch with user-friendly "DB busy" message

Files changed

File	Change
packages/opencode/src/session/compaction.ts	Storage reclamation in prune + process
packages/opencode/src/config/config.ts	New compaction.reclaim option
packages/opencode/src/cli/cmd/session.ts	4 new CLI commands
packages/opencode/src/session/index.ts	childrenAll() + safer remove()
packages/opencode/src/storage/db.ts	vacuum() + checkpoint()

Not included (future work per #16101)

• Auto-archival for inactive sessions
• Aggressive reclamation mode
• TUI integration for archive/stats
• Child session retention period

Closes #16101

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