chore(prompts): align model tags and add report plan

- Update prompt model frontmatter in code-review and feature-add prompts to Claude Sonnet 4.6 (copilot)
- Add a save_report implementation plan prompt to support next-session delivery and clearer handoff context

.github/prompts/code-review.prompt.md

@@ -2,7 +2,7 @@
 agent: Plan
 name: code-review
 description: This prompt helps you review code for your existing MCP server by guiding you through logic analysis, security checks, and performance assessments.
-model: Claude Opus 4.6
+model: Claude Sonnet 4.6 (copilot)
 ---
 <system_role>
 You are a Senior Principal Engineer and Enterprise Architect. You specialize in MCP (Model Context Protocol) and high-integrity data synchronization. Your goal is to review code for a non-technical founder managing "information drift" across enterprise systems.

.github/prompts/feature-add.prompt.md

@@ -2,7 +2,7 @@
 agent: Plan
 name: feature-add
 description: This prompt helps you add a new feature to your existing MCP server by guiding you through branch creation, code drafting, and deployment steps.
-model: Claude Opus 4.6
+model: Claude Sonnet 4.6 (copilot)
 ---
 
 <system_role>

.github/prompts/plan-saveReportTool.prompt.md

@@ -0,0 +1,66 @@
+# Plan: Add `save_report` Tool to nexus-mcp
+
+## TL;DR
+Add a new `save_report` MCP tool that accepts a title and markdown content, then writes it as a `.md` file to the configured reports directory (`./documentation/output-reports/`). Implemented as a new `reports.py` shard — consistent with the existing shard pattern and aligned with the pre-staged `generate_*` tools referenced in the README. All dependencies (`aiofiles`, `ReportConfig`) are already present.
+
+---
+
+## Branch Identity
+- Branch name: `feat/add-save-report-tool`
+
+---
+
+## Steps
+
+### Phase 1 — Create the shard
+1. Create `nexus-mcp/src/shards/reports.py`
+   - Define `register(mcp: FastMCP) -> None`
+   - Inside, define `save_report(title, content, subfolder, filename)` as `@mcp.tool()` async fn
+   - Use `ReportConfig` from `lib/config.py` for `output_dir`
+   - Slugify title → safe filename; add ISO timestamp suffix to avoid collisions
+   - `mkdir(parents=True, exist_ok=True)` on resolved path
+   - Write file using `aiofiles.open(..., "w")`
+   - Return `{"status": "saved", "path": str(abs_path), "filename": final_filename, "size_bytes": N}`
+
+### Phase 2 — Register the shard in main.py
+2. In `nexus-mcp/src/main.py`:
+   - Import the new shard: `from shards import reports`
+   - Add conditional registration block (matches existing pattern): `if os.getenv("ENABLE_REPORTS", "true").lower() == "true": reports.register(mcp)`
+   - Placement: after the existing shard registrations, before audit middleware wrapping
+
+### Phase 3 — Environment config
+3. Add `ENABLE_REPORTS=true` to `nexus-mcp/.env` (if file exists; else note it as optional)
+
+---
+
+## Relevant Files
+- `nexus-mcp/src/shards/reports.py` — NEW file to create
+- `nexus-mcp/src/main.py` — add shard import + conditional registration
+- `nexus-mcp/lib/config.py` — reuse `ReportConfig` (specifically `output_dir: Path`)
+- `nexus-mcp/.env` — add `ENABLE_REPORTS=true`
+- `nexus-mcp/pyproject.toml` — no changes needed (aiofiles, jinja2, tabulate already listed)
+
+---
+
+## Verification
+1. Restart nexus-mcp server; tool list should include `save_report`
+2. Call `save_report(title="Test Report", content="# Hello\nThis is a test.")` from Claude
+3. Verify file appears at `nexus-mcp/reports/test-report-<timestamp>.md`
+4. Call `nexus_audit_recent` and confirm a `save_report` entry was logged
+5. Run existing tests: `cd nexus-mcp && python -m pytest tests/`
+
+---
+
+## Decisions
+- New shard (`reports.py`) over adding to `main.py` — keeps built-ins minimal, aligns with planned `generate_*` tools
+- `aiofiles` for async writes — already a dependency, correct for async tool context
+- Timestamp in filename — prevents silent overwrite collisions
+- Default `ENABLE_REPORTS=true` — safe; no external API calls, pure local I/O
+- Scope: only `save_report` — user requested this one tool; `generate_*` stubs are out of scope
+
+---
+
+## Out of Scope
+- Implementing planned `generate_*` tools
+- A `list_reports` or `delete_report` tool
+- Any templating (Jinja2) or formatting logic beyond saving raw markdown