Add comprehensive README for Krypto CLI agent

[Copilot]

The repository had an empty README, leaving no documentation of what Krypto is, what it can do, or how to run it.

Changes

• What it is: Describes Krypto as a terminal-based AI coding agent — autonomous agentic loop, LLM-backed, OpenAI-compatible API
• Interaction modes: Interactive REPL vs. single-shot prompt mode, -c flag for working directory scoping
• Built-in tools table: All 11 tools (shell, read_file, write_file, edit, list_dir, grep, glob, web_search, web_fetch, memory, todo) with kind and behavior notes
• Sub-agents: Built-in code_reviewer, custom sub-agent config via .krypto/config.toml
• Configuration: Two-layer TOML merge (system → project), key options documented with example
• AGENT.MD support: Per-project developer instructions auto-loaded into system prompt
• Security model: Blocked shell commands, env-var scrubbing, path validation, prompt-injection defense
• Tech stack table: Python 3.12, OpenAI SDK, Rich, Click, Pydantic v2, httpx, ddgs, platformdirs, Ruff
• Architecture tree: Maps each directory to its role
• Quick start: Minimal env setup and invocation examples

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