feat: add product project hierarchy

[siisee11]

Summary

• introduce Product as the planning layer above repository-scoped Projects
• fan out Product Work Items into per-project Hunt Runs with cross-project dependencies and aggregated status
• add Product-aware desktop, Android shared UI, native iOS grouping, API contracts, migration, and architecture docs

Verification

• bun run test (1,062 tests)
• bun run check
• bun run worker:check
• bun run mobile:contract
• bun run build
• bun run worker:build
• browser-verified multi-repository creation and sequential execution selection
• changed Swift sources pass swiftc -parse

Known existing issue

• bun run ios:native:test remains blocked by pre-existing compile errors in unchanged IssueWriteViews.swift lines 120 and 242.