[Pre-Sept 28] Extension: add metamask.com to externally_connectable in v2 Chrome manifest

### What is this about?

Key details
Description
The extension v2 Chrome manifest restricts which web pages can communicate with the extension via chrome.runtime.sendMessage(). As of Sept 2026, externally_connectable.matches is set to ["https://metamask.io/*"]. Any MetaMask web property that moves to metamask.com and uses this messaging channel will silently lose it until the manifest is updated.

File: app/manifest/v2/chrome.json

Add "https://metamask.com/*" to externally_connectable.matches

Keep "https://metamask.io/*" during the transition window

Note: the v3 Chrome manifest uses a wildcard — no change needed. Firefox manifests must not be changed — webextension@metamask.io is the permanent AMO addon identifier and cannot be updated without breaking existing Firefox installs.

Acceptance criteria:

metamask.com/* added to externally_connectable.matches in v2 Chrome manifest

Extension shipped to Chrome Web Store before Sept 28

Web properties on both metamask.io and metamask.com can message the extension

Corresponding Jira issue: https://consensyssoftware.atlassian.net/browse/DUS-15689

### Scenario

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### Design

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### Technical Details

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### Threat Modeling Framework

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### Acceptance Criteria

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### Stakeholder review needed before the work gets merged

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

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Open app/manifest/v2/chrome.json and inspect externally_connectable.matches. Add the metamask.com wildcard while retaining metamask.io, then verify the v2 manifest permits messaging from both domains and that v3 and Firefox manifests remain unchanged. The extension must be shipped to the Chrome Web Store before Sept 28.

Written by the indexing model from the issue text.