docs: add interview-focused Relay codebase guide

[Manavarya09]

Motivation

• Provide a concise, interview-friendly walkthrough of the Relay codebase so contributors can quickly explain architecture, runtime flow, and design tradeoffs.
• Surface the most important code paths and operational behaviors (capture → build/compress → safety/delivery → agents) in one place to speed onboarding and interview prep.

Description

• Added a new documentation file at docs/INTERVIEW_GUIDE.md that summarizes product purpose, architecture, end-to-end handoff flow, reliability features, storage model, extensibility, and engineering tradeoffs.
• The guide includes a 90‑second walkthrough script, a quick file map (pointing to key files like core/src/main.rs, core/src/capture/session.rs, core/src/handoff/mod.rs, core/src/scoring.rs, core/src/agents/mod.rs, core/src/watch.rs), and suggested improvements to discuss in an interview.
• No runtime code changes were made; this is a docs-only addition aimed at knowledge transfer and interview readiness.

Testing

• Ran the automated test suite for the Rust core with cd core && cargo test -q, and all tests completed successfully (all tests passed).

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Codex Task