Support passing environment variables when starting Dev Proxy

[Copilot]

• Add env property to DevProxyTaskDefinition interface in task-provider.ts and pass to ShellExecution options
• Add env property to task definition schema in package.json
• Add dev-proxy-toolkit.env configuration setting in package.json for terminal-based startup
• Update TerminalService to accept and apply environment variables when creating terminals
• Add tests for new environment variable support in task-provider and terminal tests
• Update README with configuration table and task example for env support
• Update CHANGELOG with entries for new env setting and task property
• Run code review and address feedback
• Run CodeQL security scan (no alerts)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support passing environment variables when starting Dev Proxy</issue_title>
<issue_description></issue_description>

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

• Fixes Support passing environment variables when starting Dev Proxy #223

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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 CHANGELOG entries under [1.13.0] - Unreleased for the new env setting and task definition property. README was already updated in prior commits with the configuration table entry and task example. See 239caa4.

[garrytrinder]

Closing in favour of a broader "Start with Options..." command. The env vars idea is good but should be part of an interactive launch experience rather than a standalone setting. See new issue.