feat(jira): add getActiveIssue command to expose active issue key to other extensions

[OutOfBears]

What Is This Change?

Adds a new VS Code command atlascode.jira.getActiveIssue that returns the key of the currently active Jira issue displayed in the status bar.

This allows other VS Code extensions and tasks (e.g., git commit hooks, custom task runners, or companion extensions) to programmatically retrieve the active Jira issue key without scraping the UI. The command is registered in the JiraActiveIssueStatusBar and simply returns this.activeIssue?.key, so it's lightweight and has no side effects.

Changes across 5 files:

• package.json — Registers the new atlascode.jira.getActiveIssue command under the "Jira" category.
• src/constants.ts — Adds the JiraGetActiveIssue entry to the Commands enum.
• src/views/jira/activeIssueStatusBar.ts — Imports the new command constant and registers a handler via commands.registerCommand that returns the active issue's key (or undefined if none is set).
• src/views/jira/activeIssueStatusBar.test.ts — Adds a test verifying the command is registered and returns the expected issue key.
• CHANGELOG.md — Documents the new command in the Unreleased section.

How Has This Been Tested?

• Added a unit test in activeIssueStatusBar.test.ts that confirms the command is registered and returns the correct issue key ('TEST-123') when an active issue is present.
• The implementation is a single-line return from an already-maintained property (this.activeIssue?.key), so risk of regression is minimal.

Basic checks:

• npm run lint
• npm run test

Advanced checks:

• If Atlassian employee & Bitbucket changes: did you test with DC in mind? [See Instructions](https://www.loom.com/share/71e5d17734a547f68fd6128be6cd760e?sid=835e58a7-1240-498d-b2d7-fa7fdf8ffa36)

Recommendations:

• Update the CHANGELOG if making a user facing change

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