docs(#753): fix mission FK candidate key

docs/scratchpads/753-kbn010-threat-gate.md

@@ -53,3 +53,23 @@ Pending worker validation, independent SecReview, Ultron gate, PR merge, post-me
 - Independent SecReview remains pending and cannot return PASS until contract authority resolves `KBN010-SI-001`.
 - Final formatting/diff/test-ID completeness review — PASS (all catalog prefixes contiguous with no duplicate IDs).
 - Remaining: scoped commit, queue guard, and push.
+
+## 2026-07-14 KBN010-SI-001 contract-authority amendment
+
+- Authority: `web1:mosaic-100` directed the minimal rc.4 amendment under issue #753: add a non-partial unique candidate key on `missions(workspace_id, id)` while retaining global `missions.id` uniqueness and the project-congruent `(workspace_id, project_id, id)` key.
+- Rationale: artifacts and approval decisions are polymorphic exactly-one-target records and do not consistently carry `project_id`; widening both children would broaden frozen v1 semantics without improving tenant safety.
+- Plan: amend only the frozen schema contract and shared contract, freeze exact DDL ordering and future migration negatives, run scoped/full validation and independent review, then commit and push without opening/merging a PR or closing #753.
+- TDD: not applicable because this is a design-contract amendment with no runtime schema or migration implementation; rc.4 freezes future executable empty/prod/N-1/rollback/foreign-workspace and duplicate-key-feasibility tests.
+- Budget: no explicit cap supplied; use a 20K-equivalent soft working cap and one bounded worker lane.
+- Read-only #757 boundary: PR #757 adds separate logical-agent connector lease/CAS fencing (`logical_agent_connector_leases.lease_epoch`) in runtime schema and connector contracts. SI-001 changes only the frozen mission candidate key; it does not consume, alter, or reinterpret connector fencing, task fencing, lease authority, or #757 ownership.
+
+### Amendment verification evidence
+
+- Frozen schema: added exactly one non-partial `missions_workspace_id_uidx` on `(workspace_id, id)`; retained the global `id` primary key and `missions_workspace_project_id_uidx` on `(workspace_id, project_id, id)`.
+- FK reconciliation: targeted static checks prove the candidate key precedes both `artifacts_workspace_mission_fk` and `approval_decisions_workspace_mission_fk`; each continues to reference exact ordered columns `(workspace_id, mission_id)` → `missions(workspace_id, id)` with RESTRICT deletion.
+- Shared contract: candidate version is `1.0.0-rc.4`; authority, rationale, exact effect/non-effect, candidate-before-FK DDL order, duplicate feasibility, empty/prod/N-1/rollback/foreign-workspace future tests, and unchanged KCR/SOT/tenant/proposal/fencing/no-cascade invariants are explicit.
+- Changed-file Prettier, contract ESLint, `pnpm exec tsc -p docs/native-kanban-sot/tsconfig.json`, and targeted SI-001 static checks — PASS.
+- Full `pnpm format:check && pnpm lint && pnpm typecheck` — PASS (23 lint tasks and 42 typecheck/build tasks).
+- Independent Codex security review — PASS, zero critical/high/medium/low findings; tenant isolation is preserved.
+- Independent Codex code review confirmed the candidate key repairs both FK targets and reported no blocker on the authorized delta. Its initial request to rewrite the historical KBN-010 verdict conflicts with the exclusive scope and was resolved by documenting the immutable-evidence boundary in rc.4; its remaining finding concerns pre-existing live `.mosaic` session files, which are untouched and excluded from this commit.
+- Scoped diff: only the two frozen contract files and this append-only scratchpad amendment are staged for delivery; no runtime schema, migration, task plan, gate verdict, provider artifact, or #757-owned file is included.