Add development docs for FastMCP (#1719)

Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>
Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com>

AGENTS.md

@@ -48,6 +48,15 @@ When modifying MCP functionality, changes typically need to be applied across al
 - **Resource Templates** (`src/resources/` + `ResourceManager`)
 - **Prompts** (`src/prompts/` + `PromptManager`)
 
+## Writing Style
+
+- Be brief and to the point. Do not regurgitate information that can easily be gleaned from the code, except to guide the reader to where the code is located.
+- **NEVER** use "This isn't..." or "not just..." constructions. State what something IS directly. Avoid defensive writing patterns like:
+  - "This isn't X, it's Y" or "Not just X, but Y" → Just say "This is Y"
+  - "Not just about X" → State the actual purpose
+  - "We're not doing X, we're doing Y" → Just explain what you're doing
+  - Any variation of explaining what something isn't before what it is
+
 ## Testing Best Practices
 
 ### Testing Standards
@@ -59,6 +68,10 @@ When modifying MCP functionality, changes typically need to be applied across al
 - **NEVER** add `@pytest.mark.asyncio` to tests - `asyncio_mode = "auto"` is set globally
 - **ALWAYS** run pytest after significant changes
 
+### Inline Snapshots
+
+FastMCP uses `inline-snapshot` for testing complex data structures. On first run with empty `snapshot()`, pytest will auto-populate the expected value. To update snapshots after intentional changes, run `pytest --inline-snapshot=fix`. This is particularly useful for testing JSON schemas and API responses.
+
 ### Always Use In-Memory Transport
 
 Pass FastMCP servers directly to clients for testing:
@@ -203,6 +216,7 @@ If something needs work, your review should help it get there through specific,
 ### Review Checklist
 
 Before approving, verify:
+
 - [ ] All required development workflow steps completed (uv sync, pre-commit, pytest)
 - [ ] Changes align with repository patterns and conventions
 - [ ] API changes are documented and backwards-compatible where possible