P4: contrib/store — persona SQLite store (JSON-blob round-trip)

db.Personas() satisfies persona.Storage over SQLite. Each Agent is stored as a
JSON blob with extracted indexed columns (owner_id, name, webhook_secret,
chatbot_channel_filter, schedule, next_run_at) — so the WHOLE struct round-trips
(no domain<->GORM<->DB field-loss footgun) while the lookups stay indexable.

Test proves the round-trip preserves nested + map fields (SkillPalette,
StateReactEmoji), the owner/name + webhook + chatbot-filter queries, the
scheduled-due query, and MarkAgentScheduledRun clearing the due window.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

contrib/store/go.sum

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