Use update manifest for extension setup

[Nek-12]

Summary

• stop packaging extension artifacts inside the npm package
• resolve setup extension install URLs from the remote Firefox update manifest with exact CLI-version matching
• move release checks to validate update-manifest entries and signed XPI payloads against dist/extension

Verification

• bun run typecheck
• bun run lint:eslint
• bun run test packages/cli/src/cli-setup-doctor.test.ts scripts/test/package-check.test.ts scripts/test/prepare-release-version.test.ts scripts/test/package-signed-extension.test.ts
• bun run build:artifacts && bun run package:verify
• node dist/package/bin/firefox-cli.js setup --json
• bun run check: initial pass succeeded before review fixes; final rerun hit transient process-runner kill EPERM, and immediate rerun of scripts/test/process-runner.test.ts passed

Note: AMO_SOURCE_REVIEW.md was already deleted in the worktree before this branch and is intentionally not included.

[Nek-12]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

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