Add automated install and upgrade support for Linux

[Copilot]

• Add installOnLinux() function in src/commands/install.ts that uses the official install script (https://aka.ms/devproxy/setup.sh for stable, https://aka.ms/devproxy/setup-beta.sh for beta) via bash -c "$(curl -sL <url>)"
• Check for curl and bash availability before running the install script (shared checkLinuxPrerequisites() helper)
• Extract buildLinuxInstallCommand() helper for constructing the shell command
• Update upgradeDevProxy() to support Linux upgrade using the install script (instead of redirecting to docs)
• Add getInstallScriptUrl() helper in src/utils/shell.ts to return the correct URL based on version preference
• Add Linux setup script URLs (linuxSetupScript, linuxSetupBetaScript) to src/constants.ts
• Export getInstallScriptUrl from src/utils/index.ts barrel
• Add tests for getInstallScriptUrl() in src/test/shell.test.ts
• Update notification test description for Linux (no longer "unsupported")
• Add changelog entry under [1.13.0] - Unreleased
• Update README Quick Start to mention Linux automated install support
• Validate changes compile and pass lint
• Code review feedback addressed
• CodeQL security check passed (0 alerts)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add install support for Linux</issue_title>
<issue_description>We have an install script to install Dev Proxy on Linux we can use this in the toolkit to automate the install.

See https://learn.microsoft.com/microsoft-cloud/dev/dev-proxy/get-started/set-up?tabs=automated&pivots=client-operating-system-linux</issue_description>

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

• Fixes Add install support for Linux #217

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[garrytrinder]

@copilot update this PR to include changelog and readme updates following the guidelines in AGENTS.md

[Copilot]

@copilot update this PR to include changelog and readme updates following the guidelines in AGENTS.md

Added in fe0f1a5:

• CHANGELOG.md: Added entry under [1.13.0] - Unreleased → Added: section with Install: prefix
• README.md: Updated Quick Start to mention automated install support on Windows (winget), macOS (Homebrew), and Linux (setup script)