Add card linking feature (dev.skyboard.link)

[disnet]

Summary

• Add new dev.skyboard.link AT Protocol record type for directional task-to-task relationships
• Support three link types: blocks, relates to, duplicates
• Full-stack implementation across frontend (Svelte), appview (Bun/SQLite), and AT Protocol lexicons

Changes

Data layer:

• New dev.skyboard.link lexicon definition
• Link/LinkRecord types and LINK_COLLECTION constant in types.ts
• Dexie table (version 11) with indexes on boardUri, sourceTaskUri, targetTaskUri, syncStatus
• CRUD helpers in links.ts (createLink, deleteLink)

Sync & Auth:

• Link sync to PDS in syncPendingToPDS, deleteBoardFromPDS, deleteLinkFromPDS, resetErrorsToPending
• OAuth scope updated with repo:dev.skyboard.link

Appview:

• SQLite schema with links table and boardUri index
• LinkRow interface + upsert/get/delete functions in client.ts
• LinkRecordSchema Zod validation
• Firehose processor handles link create/delete events
• Links included in BoardResponse from get-board API

Frontend UI:

• Board page: allLinks liveQuery, linksByTask/linkCountsByTask derived values, link sync error reauth banner
• TaskCard: link count badge with chain-link icon
• TaskEditModal: full Links section with add link form (type selector + task picker), directional labels (blocks/blocked by, relates to, duplicates/duplicated by), and delete button

Test plan

• Verify type check passes (npm run check)
• Verify build succeeds (npm run build)
• Create links between tasks and verify they appear on both source and target cards
• Test all three link types (blocks, relates to, duplicates)
• Verify directional labels display correctly (blocks vs blocked by)
• Verify link deletion works for link author and board owner
• Verify links sync to PDS and appear via WebSocket updates
• Verify reauth banner appears on link sync errors

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