feat(rtk): add RTK devcontainer feature

[pocky]

Summary

• Add RTK as a new Dev Container feature, enabling token-optimized CLI proxying (60-90% savings) for AI coding agents inside devcontainers
• Supports latest or pinned version installs, with automatic architecture detection (amd64/arm64) and global RTK initialization for the container user
• Document the new feature in README alongside existing features (Flutter, Claude Code, Tree-sitter, GrepAI)

Changes

Feature Implementation

• src/rtk/devcontainer-feature.json: Feature manifest declaring RTK with a single version option, runs after common-utils
• src/rtk/install.sh: Install script resolving latest version via GitHub API, downloading arch-specific musl/gnu binary, installing to /usr/local/bin, and running rtk init --global for the container user

Tests

• test/rtk/scenarios.json: Default test scenarios configuration (empty, uses devcontainer feature test defaults)
• test/rtk/test.sh: Smoke tests verifying rtk is on PATH and rtk --version returns a non-empty string

Documentation

• README.md: Add RTK section with usage example, version option table, architecture support note, and global init behavior; update TOC and repository structure diagram

Test plan

• Build the feature locally: devcontainer features test -f rtk .
• Verify rtk is on PATH inside the container: command -v rtk
• Verify rtk --version returns a valid version string
• Test with a pinned version option ("version": "0.22.2") to confirm version resolution works

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