docs(rfc): propose Git repository understanding for PreparedContext

[Teingi]

Which issue or RFC does this PR close?

Introduces RFC 1619, building on #1489 (PreparedContext text assembly):

• English RFC
• Chinese RFC

Rationale for this change

Coding tasks need both current code facts and historical constraints. This proposal lets prepare_context
retrieve relevant code and deliver it with existing context under one budget, reducing repeated navigation
and manual context assembly.

What changes are included in this PR?

• English and Chinese RFCs organized using the standard nine-section template.
• An opt-in include_code preparation option, a replaceable CodeGraph adapter, and one local Python repository
  configured per existing Scope, with shared byte and entry budgets.
• Index/content consistency, authorization, fallback behavior, independent read-only queries, optional Source
  capture, and task-based acceptance criteria.
• Existing Artifact families remain intact; the proposed integration uses rebuildable local caches without
  adding Code Artifacts or business database tables.

Are there any user-facing changes?

Documentation only. The RFC proposes behavior and API changes; this PR does not implement them or require a
data migration.

How was this change tested?

• make docs-test: passed on the final numbered documents, including website tests, link validation,
  compilation, and static export verification of 808 public pages.
• Verified matching bilingual heading hierarchy, tables, numeric policy values, configuration, commands, API
  examples, and diagram flow. Checked local links and the required content exclusions.
• git diff --cached --check: passed.
• .venv/bin/prek run -a: formatting and lint hooks passed. ty check reports three unresolved
  scripts.integration_guidance_* / scripts.evaluate_integration_guidance imports in
  tests/test_integration_guidance_evaluation.py. The same three errors were reproduced in an unmodified
  upstream checkout with the same Python environment. Only this known baseline check is skipped by the commit
  hook.

AI usage statement

OpenAI Codex (GPT-6) assisted with drafting, translation, editing, and validation of these RFC documents.