chore: enhance planning and management instructions with PR slicing strategies and multi-PR protocols

.github/instructions/copilot-instructions.md

@@ -123,6 +123,21 @@ Before proposing ANY code change or fix, you must build a mental map of the feat
 - **Beta**: `feature/beta-release` always builds.
 - **History-Rewrite PRs**: If a PR touches files in `scripts/history-rewrite/` or `docs/plans/history_rewrite.md`, the PR description MUST include the history-rewrite checklist from `.github/PULL_REQUEST_TEMPLATE/history-rewrite.md`. This is enforced by CI.
 
+## PR Sizing & Decomposition
+
+- **Default Rule**: Prefer smaller, reviewable PRs over one large PR when work spans multiple domains.
+- **Split into Multiple PRs When**:
+    - The change touches backend + frontend + infrastructure/security in one effort
+    - The estimated diff is large enough to reduce review quality or increase rollback risk
+    - The work can be delivered in independently testable slices without breaking behavior
+    - A foundational refactor is needed before feature delivery
+- **Suggested PR Sequence**:
+    1. Foundation PR (types/contracts/refactors, no behavior change)
+    2. Backend PR (API/model/service changes + tests)
+    3. Frontend PR (UI integration + tests)
+    4. Hardening PR (security/CI/docs/follow-up fixes)
+- **Per-PR Requirement**: Every PR must remain deployable, pass DoD checks, and include a clear dependency note on prior PRs.
+
 ## ✅ Task Completion Protocol (Definition of Done)
 
 Before marking an implementation task as complete, perform the following in order:

.github/instructions/subagent.instructions.md

@@ -23,10 +23,22 @@ runSubagent({
 
 - Validate: `plan_file` exists and contains a `Handoff Contract` JSON.
 - Kickoff: call `Planning` to create the plan if not present.
+- Decide: check if work should be split into multiple PRs (size, risk, cross-domain impact).
 - Run: execute `Backend Dev` then `Frontend Dev` sequentially.
 - Parallel: run `QA and Security`, `DevOps` and `Doc Writer` in parallel for CI / QA checks and documentation.
 - Return: a JSON summary with `subagent_results`, `overall_status`, and aggregated artifacts.
 
+2.1) Multi-PR Slicing Protocol
+
+- If a task is large or high-risk, split into PR slices and execute in order.
+- Each slice must have:
+  - Scope boundary (what is included/excluded)
+  - Dependency on previous slices
+  - Validation gates (tests/scans required for that slice)
+  - Explicit rollback notes
+- Do not start the next slice until the current slice is complete and verified.
+- Keep each slice independently reviewable and deployable.
+
 3) Return Contract that all subagents must return
 
 ```
@@ -43,6 +55,7 @@ runSubagent({
 
 - On a subagent failure, the Management agent must capture `tests.output` and decide to retry (1 retry maximum), or request a revert/rollback.
 - Clearly mark the `status` as `failed`, and include `errors` and `failing_tests` in the `summary`.
+- For multi-PR execution, mark failed slice as blocked and stop downstream slices until resolved.
 
 5) Example: Run a full Feature Implementation