docs: update README features to reflect current implementation

[chaliy]

Summary

• Updated main README feature list from 9 to 15 items, adding security-first positioning, RealFs, LLM tool contract, scripted tool orchestration, MCP server, language bindings, multi-tenant isolation
• Expanded builtin commands table from ~80 to all 150 commands with new categories (Checksums, Data formats, DevOps)
• Expanded shell features section from 9 to 20 items (extended globs, namerefs, assoc arrays, process substitution, coprocesses, brace expansion, pipefail, etc.)
• Added expanded Security section with Defense in Depth table covering all 10 security layers
• Rewrote JS crate README with full API surface: VFS methods, async execute, cancellation/AbortSignal, ScriptedTool, BashError, file mounts, BashOptions
• Updated Python crate README with execute_or_throw variants, full FileSystem API, Deep Agents integration, ScriptedTool missing methods
• Added Bun and Deno support mentions across all READMEs
• Fixed CLI binary name from bashkit-cli to bashkit
• Added all feature flags to install section (git, python, realfs, scripted_tool)
• Added MCP server and RealFs mount flags to CLI usage section

Test plan

• cargo test --features http_client — all 2900+ tests pass
• just check (fmt + clippy + test) — passes
• No code changes — docs only, no functional impact