docs(#769): start KBN-100 schema migration

docs/scratchpads/769-kbn100-schema-migration.md

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+# Issue #769 — KBN-100 unified schema and N-1 migration
+
+## Status
+
+IN PROGRESS — tracking and baseline gate. No schema or migration implementation is authorized until this scratchpad and `docs/native-kanban-sot/TASKS.md` are committed/pushed and baseline evidence is recorded.
+
+## Requirement authority
+
+- Provider issue: <https://git.mosaicstack.dev/mosaicstack/stack/issues/769>
+- Canonical requirements: `docs/requirements/native-kanban-sot.md`
+- Frozen integration contract: `docs/native-kanban-sot/SHARED-CONTRACT.md` v1.0.0-rc.4
+- Frozen target schema: `docs/native-kanban-sot/contracts/kanban-schema.v1.ts`
+- Threat/auth/constraint gate: `docs/native-kanban-sot/KBN-010-THREAT-AUTH-CONSTRAINT-GATE.md`
+- Dependency evidence: PR #765 merge `2e228007`, issue #753 closed, post-main pipeline #1813 terminal success.
+
+## Fixed decisions
+
+1. PostgreSQL is the sole writable project/task/orchestration SOT; generated projections and external transports are never import/write authority.
+2. Expand is additive and N-1-safe. Do not drop, rename, narrow, or reinterpret legacy data during this card.
+3. Backfill is bounded, idempotent, resumable, checksummed, and quarantines ambiguity rather than guessing.
+4. rc.4 SI-001 creates `missions_workspace_id_uidx(workspace_id,id)` before both dependent artifact/approval FKs while retaining global and project-congruent keys.
+5. Runtime repositories, Gateway/MCP, Coordinator behavior, clients, fleet, channels, connector fencing, deployment, and production migration execution are out of scope.
+6. Critical schema/data behavior uses TDD RED/GREEN and real PostgreSQL integration evidence; PGlite-only evidence cannot certify the migration.
+
+## Authorized paths
+
+- `packages/db/src/schema.ts`
+- `packages/db/drizzle/**`
+- DB-package tests/config files explicitly enumerated by the author before first edit
+- `docs/native-kanban-sot/TASKS.md`
+- this scratchpad
+
+Any additional path requires control-plane approval before edit.
+
+## Delivery cycle
+
+`baseline -> test design/RED -> schema+migration GREEN -> focused tests -> real PostgreSQL migration matrix -> full gates -> independent DB review -> SecReview -> remediate/rereview -> Codex-Ultron -> PR CI -> squash merge -> post-main CI -> issue close`
+
+## Baseline evidence
+
+Pending fresh author-lane intake and baseline test run.
+
+## Collision boundaries
+
+- #758 owns generic local Fleet configuration/lifecycle cards.
+- #757/#755 owns logical-agent connector identity/lease/fencing.
+- #756 channel foundation is merged and remains separate.
+- KBN-105 and all KBN consumers remain held until #769 completes.
+
+## Completion evidence
+
+Pending implementation, reviews, PR, merge, terminal-green main CI, and issue closure.