chore: add build:chrome, build:firefox, build:all unpacked dev-directory builds

[Ayush7614]

Implements the remaining TODOs.md #5 dev-install story: deterministic unpacked development directories built from the working tree.

• New scripts/build-unpacked.mjs (ESM, no deps): resolveBuildPlan + assertDevBuildInputs pure helpers, --browser/--out-dir/--dry-run/--clean flags, manifest version-match validation, prints exact chrome://extensions and about:debugging load paths. Working tree is the source of truth (unlike build-zip HEAD archiving); never for store publishing.
• package.json: build:chrome, build:firefox, build:all scripts.
• .gitignore: ignore build/ outputs.
• README: document npm run build:chrome/build:firefox alternative load paths.

Verification:

• node --check scripts/build-unpacked.mjs
• --dry-run plus real npm run build:all, both manifests exist and versions match package.json 36.0.4, cleaned up afterwards
• node test/run.js: 2298 passed, 0 failed

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