feat(agent): add aider.chat as a fourth agent backend

[igmarin]

What shipped:

New src/aider.rs runner: message-mode aider with per-session --chat-history-file, --restore-chat-history on resume, SessionMissing on missing history, SOUL.md attached read-only via --read, stderr never leaked into errors, and session-id path-escape guards.
AgentBackend::Aider wired through config.rs, agent.rs, gateway/mod.rs, main.rs, test helpers, and docs.
All checks green locally: fmt, clippy -D warnings, build, 254 unit + integration tests, mkdocs build --strict, tests/install.sh.

Two notes:

1. Building needs Rust with edition2024 support (I used stable 1.97; VM shipped 1.83); upstream CI uses latest stable so this only affects local builds.
2. Live end-to-end testing would need a real aider install + model-provider API key.