browser-firefox dry-run builds should emit a package plan

[RasputinKaiser]

The browser-firefox target currently logs a web-ext build step and returns a guessed zip path, but dry-run builds do not leave behind a concrete artifact that callers can inspect.

A Firefox package-plan artifact would make dry runs safer and more useful by recording the resolved source directory, expected AMO artifact, channel, and the exact web-ext build command without touching source files or requiring AMO credentials.

[Robertg761]

Implemented in PR #180: #180

Summary:

• dry-run browser-firefox builds now write firefox-package-plan.json with provider, extensionId, version, sourceDir, expectedArtifact, channel, and the resolved web-ext build argv command
• dry-run ships remain side-effect free and expose resolved source/channel metadata
• real-build artifact path behavior is preserved with command metadata for callers

Verification:

• pnpm exec vitest run packages/targets/browser-firefox/src/index.test.ts
• pnpm --filter @profullstack/sh1pt-target-browser-firefox typecheck
• pnpm --filter @profullstack/sh1pt typecheck
• git diff --check