refactor: remove legacy demo code, integrate metrics via pipeline context

- Remove ~700 lines of legacy code from app.py (run_demo_mode, run_pipeline_demo,
  run_preset_mode, font picker, effects picker)
- HUD now reads metrics from pipeline context (first-class citizen) with fallback
  to global monitor for backwards compatibility
- Add validate_signal_flow() for PureData-style type validation in presets
- Update MicSensor documentation (self-contained, doesn't use MicMonitor)
- Delete test_app.py (was testing removed legacy code)
- Update AGENTS.md with pipeline architecture documentation

AGENTS.md

@@ -161,7 +161,7 @@ The project uses pytest with strict marker enforcement. Test configuration is in
 
 ### Test Coverage Strategy
 
-Current coverage: 56% (336 tests)
+Current coverage: 56% (433 tests)
 
 Key areas with lower coverage (acceptable for now):
 - **app.py** (8%): Main entry point - integration heavy, requires terminal
@@ -192,6 +192,47 @@ Performance regression tests are in `tests/test_benchmark.py` with `@pytest.mark
 - **effects/** - plugin architecture with performance monitoring
 - The render pipeline: fetch → render → effects → scroll → terminal output
 
+### Pipeline Architecture
+
+The new Stage-based pipeline architecture provides capability-based dependency resolution:
+
+- **Stage** (`engine/pipeline/core.py`): Base class for pipeline stages
+- **Pipeline** (`engine/pipeline/controller.py`): Executes stages with capability-based dependency resolution
+- **StageRegistry** (`engine/pipeline/registry.py`): Discovers and registers stages
+- **Stage Adapters** (`engine/pipeline/adapters.py`): Wraps existing components as stages
+
+#### Capability-Based Dependencies
+
+Stages declare capabilities (what they provide) and dependencies (what they need). The Pipeline resolves dependencies using prefix matching:
+- `"source"` matches `"source.headlines"`, `"source.poetry"`, etc.
+- This allows flexible composition without hardcoding specific stage names
+
+#### Sensor Framework
+
+- **Sensor** (`engine/sensors/__init__.py`): Base class for real-time input sensors
+- **SensorRegistry**: Discovers available sensors
+- **SensorStage**: Pipeline adapter that provides sensor values to effects
+- **MicSensor** (`engine/sensors/mic.py`): Self-contained microphone input
+- **OscillatorSensor** (`engine/sensors/oscillator.py`): Test sensor for development
+
+Sensors support param bindings to drive effect parameters in real-time.
+
+### Preset System
+
+Presets use TOML format (no external dependencies):
+
+- Built-in: `engine/presets.toml`
+- User config: `~/.config/mainline/presets.toml`
+- Local override: `./presets.toml`
+
+- **Preset loader** (`engine/pipeline/preset_loader.py`): Loads and validates presets
+- **PipelinePreset** (`engine/pipeline/presets.py`): Dataclass for preset configuration
+
+Functions:
+- `validate_preset()` - Validate preset structure
+- `validate_signal_path()` - Detect circular dependencies
+- `generate_preset_toml()` - Generate skeleton preset
+
 ### Display System
 
 - **Display abstraction** (`engine/display/`): swap display backends via the Display protocol