docs(contexts): organize design guidance for agents

[keli-wen]

Summary

Creates one place for QuantMind design guidance and explains the planned Paper extraction process before further Paper or PageIndex implementation.

• Adds a single design index with direct package and feature folders.
• Adds a plain-language Quick Summary and complete Contents preview to every context page. Agents read the first 80 lines to choose a page, then read the full page when it applies.
• Rewrites all 10 context pages with concrete headings, shorter sentences, and explanations for necessary technical terms.
• Defines Paper extraction from input to a validated Paper. A model or PageIndex may suggest sections; code creates the final IDs, tree links, page ranges, citations, and source-backed text.
• Keeps PageIndex optional behind a small PaperSourceDocument -> draft interface.
• Moves the existing News and operation-naming guidance into the design index.
• Replaces the obsolete generic storage design with the current local knowledge library: stored knowledge is preserved, while search text and embeddings can be rebuilt.
• Adds a fixed four-page test PDF and offline tests for page anchors, tree shape, validation rules, and overlapping sibling page ranges.
• Updates repository, contributor, catalog, and usage links and removes the superseded docs/design/ copies.

Related Issue

Closes #115

Verification

• ./scripts/verify.sh — 323 tests passed, 86.47% coverage, 6 import rules kept.
• Rendered and visually inspected all four pages of tests/fixtures/paper/golden/paper.pdf.
• No live-network behavior changed, so no component live check applies.
• Pre-commit and pre-push hooks passed without bypassing checks.

Checklist

• The title uses English Conventional Commit format: type(scope): summary.
• The related issue or design discussion is linked when applicable.
• bash scripts/verify.sh passes.
• Every applicable live-network check passes, or this PR states why none applies.
• Public behavior has focused tests, an example, and documentation where applicable.
• The PR is complete and contains no unrelated changes.