docs: make quick start prove first memory

[cyfyifanchen]

Summary

• Reworks the README Quick Start around a first-time developer path: prerequisites, install, configure, start, health check, first memory write, flush, and search.
• Adds a root .env.example so source-checkout users can copy a visible config file with cp .env.example .env.
• Keeps .env.example synchronized with the packaged src/everos/templates/env.template through scripts/check_docs.py.
• Adds a GitHub-rendered success callout after the first memory search so new users get a clear “core loop completed” moment.
• Moves Use Cases directly underneath Quick Start and adds a short bridge from the first successful EverOS moment into community and integration examples.
• Cross-verifies the README Quick Start, QUICKSTART.md, and README.zh-CN.md against the current CLI/API contracts.
• Corrects the internal quickstart search response shape to include unprocessed_messages, and clarifies that cascade indexing can catch up after the synchronous Markdown write.
• Removes everos --help from the main success path to reduce friction.
• Keeps multimodal ingestion and contributor setup after the core loop.
• Mirrors the README Quick Start and Use Cases ordering in README.zh-CN.md, and updates QUICKSTART.md with the copy option.

Why

The previous Quick Start got users to a running server, but did not show the first successful product loop. A new developer now sees how to create config, write a memory, search it back, understand why that moment matters, and immediately discover what people are building with persistent memory.

Verification

• uv run python <quickstart contract smoke>
  • checks README / QUICKSTART.md / README.zh-CN.md contain the documented install, config, server, add, flush, and search steps
  • validates .env.example equals the packaged src/everos/templates/env.template
  • validates everos init --to <tmp>/.env through the Typer CLI and confirms 0600 permissions
  • validates /health on create_app(lifespan_providers=[])
  • validates the documented add / flush / search payloads against the current request DTOs
  • validates the default /search response data arrays, including unprocessed_messages
• Real uv run everos server start --env-file <tmp> --port <random> health smoke
• git diff --check
• make docs-check
• uv run pytest tests/unit/test_memory/test_search/test_dto.py -v
• make test (1211 passed, 1 skipped)