plugin-sharing: INFO line when isSystem writes cover an active sharing rule but materialise zero grants (demand 3 of #4707, maintainer-ruled 2026-08-06) #6783

Part of #4707. Execution sub-card cut by the triage seat per the maintainer's 2026-08-06 ruling recorded on that thread (demand 3 = do).

Task

When a seed run (or any isSystem write batch) writes rows to an object covered by an active sharing rule and materialises zero grants because the record-write hooks skip isSystem sessions, emit one INFO line naming the behaviour and the remedy, per the ruling's wording: include the guidance "sharing materialisation skipped for isSystem writes; re-evaluate rules or restart to backfill".

Anchors (re-verified on origin/main @ e1e7629)

• The skip itself: packages/plugins/plugin-sharing/src/rule-hooks.ts:157, :165, :180, :194 — each record-write hook returns early on (ctx?.session as any)?.isSystem.
• The eventual heal the INFO line should point at: the boot backfill (sharing-plugin.ts, whose own comment at :47 documents that seed rows are written with isSystem and the hooks deliberately skip them).
• The original symptom (hotcrm#640): fresh install, 9 active rules, 9 matching accounts, sys_record_share empty — every visible layer said "configured", nothing said "inert".

Shape notes for the dev (from the parent card, not a ruling)

• The signal must be one line per seed run/batch, not per row — the failure mode being fixed is silence, not verbosity.
• Behaviour is otherwise correct (boot backfill heals it); ⛔ do not change the skip itself, and do not make the INFO an error/warn — the ruling asked for INFO.
• Demand 2 of the parent is decided (the flag is NOT being split); this card must not introduce any new switch.

Scope

packages/plugins/plugin-sharing only (identity lane).

Refs: #4707 (anchor card — closure rides on the sub-cards landing), hotcrm#640 / PR hotcrm#652, sibling devx sub-card for demand 1.