docs: improve README setup and usage

[owainlewis]

Summary

• add an end-to-end quickstart with prerequisites, release and source installation, assistant setup, and first run
• document common chat commands and job workflows
• add concise contributor checks while preserving the product overview and canonical documentation links

Why

The README explained the product well but delegated most setup and usage details to the documentation site. This gives new users a concrete path from installation to a working gateway and makes common operations easier to find.

Test plan

• cargo fmt --all --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked
• cargo test --locked (245 unit tests and 11 integration tests)
• cargo build --locked --release
• verified the documented source install into a fresh destination
• bash tests/install.sh
• mkdocs build --strict
• git diff --check
• checked 14 local README links and all external README links
• inspected the rendered branch README on GitHub for structure, overflow, and console errors
• independent subagent diff review completed; fixed its source-install finding and re-reviewed cleanly

Risks

Documentation-only change. A live Telegram or iMessage gateway was not started because that requires external account and platform state. Browser screenshots and a narrow viewport check were unavailable for the GitHub file view, but the rendered DOM had no page overflow or console errors.

Related issue

None.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Add Git to the quickstart prerequisites

For users installing the prebuilt release on a machine without git, following this Quickstart reaches push init, which unconditionally executes git init in src/assistant.rs, so setup fails even though all listed requirements are satisfied. Please include Git in the general requirements rather than only the source-build paragraph.

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