Add schpet-linear-cli devcontainer feature

[Copilot]

Adds a devcontainer feature that installs the linear binary — a CLI for interacting with the Linear issue tracker from the terminal.

Changes

• src/schpet-linear-cli/devcontainer-feature.json — feature metadata; exposes a version option (default: latest)
• src/schpet-linear-cli/install.sh — downloads pre-compiled binary from GitHub releases; handles .tar.xz via tar -xJ + xz-utils; supports amd64/arm64; archive path pattern: linear-${arch}-unknown-linux-gnu/linear
• src/schpet-linear-cli/NOTES.md — project link, description, installation method
• test/schpet-linear-cli/test.sh — validates linear --version exits successfully

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add schpet-linear-cli feature</issue_title>
<issue_description>Files:

src/(FeatureName)/devcontainer-feature.json
src/(FeatureName)/NOTES.md
src/(FeatureName)/install.sh (make sure to set executable bit)
test/(FeatureName)/test.sh
.github/workflows/test.yaml

Releases:

https://github.com/schpet/linear-cli/releases

Test the installation process extensively before submitting
Watch for files in releases, naming structure, etc.
Test CLI output extensively before writing the test file.

For NOTES.md:
Link to the project that is installed by the feature
Short description of what it does
Installation method
Other notes</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add schpet-linear-cli feature #155

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