docs: add developer guid and code-flow diagram

[Sharron4me]

Changes

Adds two new documentation files under docs/ aimed at engineers landing in the repo for the first time:

• docs/DEVELOPER.md — end-to-end code-flow walkthrough of Experiment.run_fit(), covering Controller/Scheduler/Worker coordination, the SQLite tables that back it, the shared-memory checkpoint path, the IC Ops loop, process topology, the CLI surface, and a debugging cheat sheet. All references use path:line citations into rapidfireai/ so readers can jump straight to the code.
• docs/code-flow.md — companion mermaid diagram visualizing the same flow (User → Experiment → Controller → Scheduler → Workers → SHM/MLflow/DB, plus the Dispatcher/Frontend IC Ops loop).

Documentation-only. No source, configuration, dependency, or build changes. No runtime behavior is affected.

Changelog Content

Additions

• Add docs/DEVELOPER.md developer guide with end-to-end walkthrough of the training flow, DB schema, shared-memory checkpoints, IC Ops loop, and debugging guide.
• Add docs/code-flow.md mermaid diagram of the runtime architecture.

Fixes

• None.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this change manually - verified mermaid diagram renders and path:line citations resolve against the current tree on main.
• I have tested this change in the following environments:
  • Local development

Screenshots (if applicable)

Add screenshots to help explain your changes.
N/A — diagram is inline mermaid in docs/code-flow.md and renders on GitHub.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Performance Impact

None. Documentation-only change.

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Note

Low Risk
Documentation-only changes that update developer guidance and add new diagrams; no runtime or build logic is modified.

Overview
Adds two new onboarding docs: docs/DEVELOPER.md (end-to-end Experiment.run_fit() walkthrough with path:line citations) and docs/CodeFlow.md (Mermaid diagram of the same execution flow).

Updates CLAUDE.md to link to these docs, refresh architecture/DB schema references to the rapidfireai/fit + rapidfireai/evals layout, and fixes command/script paths (e.g., setup/fit/start_dev.sh, setup/bump_version.sh).

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