chore: align agent and instruction files with single-PR commit-slicing model

- Rewrote commit slicing guidance in Management, Planning, and subagent
  instruction files to enforce one-feature-one-PR with ordered logical commits
- Removed multi-PR branching logic from the execution workflow
- Prevents partial feature merges that cause user confusion on self-hosted tools
- All cross-references now use "Commit N" instead of "PR-N"

.github/agents/Planning.agent.md

@@ -38,7 +38,7 @@ You are a PRINCIPAL ARCHITECT responsible for technical planning and system desi
    - Specify database schema changes
    - Document component interactions and data flow
    - Identify potential risks and mitigation strategies
-   - Determine PR sizing and whether to split the work into multiple PRs for safer and faster review
+   - Determine commit sizing and how to organize work into logical commits within a single PR for safer and faster review
 
 3. **Documentation**:
    - Write plan to `docs/plans/current_spec.md`
@@ -46,10 +46,10 @@ You are a PRINCIPAL ARCHITECT responsible for technical planning and system desi
    - Break down into implementable tasks using examples, diagrams, and tables
    - Estimate complexity for each component
     - Add a **Commit Slicing Strategy** section with:
-       - Decision: single PR or multiple PRs
+       - Decision: single PR with ordered logical commits (one feature = one PR)
        - Trigger reasons (scope, risk, cross-domain changes, review size)
-       - Ordered PR slices (`PR-1`, `PR-2`, ...), each with scope, files, dependencies, and validation gates
-       - Rollback and contingency notes per slice
+       - Ordered commits (`Commit 1`, `Commit 2`, ...), each with scope, files, dependencies, and validation gates
+       - Rollback and contingency notes for the PR as a whole
 
 4. **Handoff**:
    - Once plan is approved, delegate to `Supervisor` agent for review.