Add Lobster Workflows guide (new, 520 lines)

Comprehensive guide for OpenClaw's official workflow shell (1,222 stars).
Covers: standalone CLI and embedded gateway plugin modes, YAML-based DSL
with 3 step types (shell, pipeline, approval), control flow (parallel,
for_each, conditionals, sub-workflows), data flow via $step references,
LLM integration with cost tracking, approval gates with identity
constraints, real-world patterns (deploy pipeline, code review chain,
overnight research, system monitoring), visualization with lobster graph,
CLI reference, environment variables, known limitations.

Closes #73

Author: melvincarvalho