feat(grepai): add GrepAI devcontainer feature

[pocky]

Summary

• Add GrepAI as a new devcontainer feature, enabling semantic code search powered by embeddings inside dev containers
• The feature installs the GrepAI CLI system-wide (via /usr/local/bin) so all container users have access, following the same pattern used by the claude-code feature
• A postStartCommand lifecycle hook displays a quickstart banner at container startup, guiding users through init, watch, search, and MCP integration with Claude Code
• Includes a full test suite verifying the binary is on PATH and reports a valid version string

Changes

Feature — src/grepai

• src/grepai/devcontainer-feature.json: Feature manifest declaring id, version, documentation URLs, and the postStartCommand pointing to the quickstart script
• src/grepai/install.sh: Installation script that ensures curl is available, runs the official GrepAI installer, relocates the binary from ~/.local/bin to /usr/local/bin for multi-user access, and installs the quickstart script
• src/grepai/grepai-quickstart.sh: Banner script run at container start with step-by-step guidance for grepai init, grepai watch, grepai search, and MCP setup for Claude Code

Tests — test/grepai

• test/grepai/scenarios.json: Defines the install_grepai_latest test scenario using the base Ubuntu devcontainer image
• test/grepai/test.sh: Verifies the grepai binary is on PATH and that grepai version returns a non-empty string
• test/grepai/install_grepai_latest.sh: Scenario entry-point script (delegates to shared test.sh checks)

Test plan

• Build the feature locally: devcontainer features test -f grepai .
• Verify grepai is accessible to all users: docker run --rm <image> which grepai
• Confirm quickstart banner appears on container start (check postStartCommand output in terminal)
• Run grepai version inside the container and confirm a valid version string is returned

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