docs: add core abstractions document

[gx-ai-architect]

Summary

• Adds docs/abstractions.md — frames the Factory from first principles
• Defines the two core primitives: The Loop (hypothesize → implement → measure → keep/revert → learn) and The Guardrails (auto-generated fitness function + non-overridable safety gates)
• Maps all 57 modules to their role (loop state, loop execution, guardrails, learning, visualization, integration)
• Classifies each as core (54%) vs optional (46%) with line counts
• Documents the key implementation choices that are debatable vs stable

This is a companion to the existing architecture.md. That doc describes how things are implemented (three layers, state machine, data flow). This doc describes what the system is — useful for onboarding, investor conversations, and deciding where to experiment.

Test plan

• Verify doc renders correctly on GitHub
• Verify module list matches current codebase
• Verify line counts are approximately correct

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.31%. Comparing base (f9bd0ce) to head (fa1c198).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #391   +/-   ##
=======================================
  Coverage   87.31%   87.31%           
=======================================
  Files          61       61           
  Lines        9339     9339           
=======================================
  Hits         8154     8154           
  Misses       1185     1185           

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