docs: modernize Hosted Web readmes

[dajiaohuang]

Summary

• rebuild the English and Simplified Chinese README entry points around the current SagaSmith Web product boundary
• document Local Kit versus Hosted Web, the exact sagasmith.release-lock/v3 component revisions, and MCP 2026-07-28 as the hosted default
• explain durable room turns, end-to-end idempotency, quota lease renewal, revision scheduling, Hosted identity/media/workspace boundaries, projection caches, and bounded Host tool selection
• provide verified development, acceptance, deployment, upgrade, rollback, observability, security, and documentation navigation guidance in both languages

Documentation evidence

• audited every tracked README (README.md and README.zh-CN.md)
• cross-checked claims and defaults against component-versions.json, .env.example, all Compose profiles, Agent configuration, API routes, migrations, implementation tests, architecture, operations, threat model, and acceptance matrix
• retained SagaSmith Web as the product name and the compatible sagasmith_service / sagasmith-service implementation identifiers
• kept archived repositories out of release inputs and described legacy MCP only as an atomic rollback path

Validation

• uv sync --frozen --all-extras
• uv run ruff check .
• uv run pytest — 201 passed, 4 skipped
• JavaScript syntax checks plus node --test tests/web/*.test.mjs — 17 passed
• docker compose config --quiet with CI-only required environment values
• local Markdown link target validation for both READMEs
• git diff --check
• README secret-pattern scan