Add Phase 1 experimenter tools: MCP server, H21cm, beacon logger, arc survey

Four new tools transforming the SkyWalker-1 from satellite TV receiver into
a general-purpose RF observatory:

- skywalker-mcp: FastMCP server exposing 20 tools, 4 resources, 2 prompts.
  Thread-safe DeviceBridge with motor safety (continuous drive opt-in),
  input validation on all frequency/symbol rate/step parameters,
  try/finally on TS capture, path traversal sanitization, and reduced
  lock scope so emergency motor halt isn't blocked during long surveys.

- h21cm.py: Hydrogen 21 cm drift-scan radiometer at 1420.405 MHz with
  Doppler velocity calculation, control band comparison, and CSV output.

- beacon_logger.py: Long-term Ku-band beacon SNR/AGC logger with auto-relock,
  dual CSV/JSONL output, signal handlers, and systemd unit generation.

- arc_survey.py: Multi-satellite orbital arc census with USALS motor control,
  per-slot catalog persistence, resume support, and defensive motor halt
  on all error/interrupt paths.

Documentation: experimenter's roadmap guide + 4 tool reference pages (48 pages total).

.gitignore

@@ -12,6 +12,10 @@ tools/__pycache__/
 tui/.venv/
 tui/__pycache__/
 
+# MCP server
+mcp/skywalker-mcp/.venv/
+mcp/skywalker-mcp/__pycache__/
+
 # Documentation site
 site/node_modules/
 site/dist/