[ENG-1855] Add Roam shared-node import discovery

[sid597]

Summary

Adds a read-only Roam command DG: Import - Discover shared nodes that lists group-visible shared nodes from other spaces — source app, source space, title, and upstream modified time — so a Roam user can see what's available to import from Obsidian/Roam. Discovery only; the import action itself is ENG-1859.

Part of the Roam↔Obsidian push-pull project, Milestone 2 (Bidirectional node import). Extends the existing Obsidian↔Obsidian discovery reference rather than building a new path.

What it does

• discoverSharedNodes reads the RLS-scoped my_contents view, excludes the current space, groups by (space_id, source_local_id), prefers the direct variant for the title, and resolves space metadata via my_spaces.
• Maps to an app-neutral shape aligned to the ENG-1847 CrossAppNode contract (sourceApp, sourceSpaceId/Name, sourceNodeId, sourceNodeRid, title, sourceModifiedAt, alreadyImported).
• Read-only Blueprint modal with loading / error / empty / list states.

Design notes (the why)

• Discovery requires a full variant (the ENG-1848 shared/published marker) so the list shows genuinely importable nodes, not every synced title — a deliberate scoping choice (diverges slightly from Obsidian's list-all discovery).
• alreadyImported flows through an empty getImportedSourceRids() seam. Roam has no imported-node store yet (that's ENG-1856), so nothing is imported and nothing is marked; ENG-1859 wires this to ENG-1856's reader. Keys on source RID.
• Visibility comes from the RLS my_* views + own-space exclusion, not app-side access logic.
• Roam-local mirror of Obsidian's getPublishedNodesForGroups / getMyGroups (app-specific client + return shape); not promoted to a shared package yet (future-extraction candidate).
• Gated behind a new Cross-app node import enabled feature flag (off by default; AdminPanel toggle "Cross-app node import"), mirroring Advanced node search enabled — the feature is incomplete (no import until ENG-1859), so it stays hidden from users by default. Later M2 commands reuse the same flag.

Testing

• Unit tests for the pure assembleDiscoveredNodes (variant merge, full-variant filter, RID round-trip, already-imported marking, sorting).
• vitest isn't installed in the local worktree (affects all existing tests) → it runs in CI; locally proven via a tsx harness (all cases pass). eslint / prettier / tsc green.

⚠️ Pending before merge

• Runtime proof against a real group-member Roam graph with another space's shared nodes in Supabase (enable the flag → run the command → confirm the list, own-space exclusion, and empty states).
• Loom walkthrough.

Linear: ENG-1855

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[linear-code]

ENG-1855

[supabase]

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
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[graphite-app]

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