docs: architecture atlas, strategy guides, and GitHub Pages

[ethancjackson]

Summary

• Add a self-contained architecture atlas and four build-phase strategy guides (guides/01–04), linked from the root README contents/docs sections.
• Wire GitHub Pages from main /docs (.nojekyll + a root redirect) so participants can open the atlas at https://vectorinstitute.github.io/agentic-forecasting/architecture-atlas.html after this merges.
• Docs-only: no library or notebook behavior changes. Accidental kernel/format noise in notebooks and analyst_agent/agent.py was left out.

Test plan

• Guide 1 smoke: register the sample CSV, cutoff context returns 523 rows ending 2025-06-02, full series is 804 rows.
• Guide 2 smoke: 13 origins × 2 horizons; naive last-value mean CRPS is 2.598 (matches the guide).
• Pre-commit hooks on the added/changed files.
• After merge, confirm https://vectorinstitute.github.io/agentic-forecasting/ and .../architecture-atlas.html render (Pages is already enabled on main /docs; the atlas lands when this merges).
• Optional: set the repo homepage to that URL once the first Pages build after merge succeeds.

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