fix(deps): move CLI provider adapters + ffmpeg-static to optionalDependencies

[AtelyPham]

Summary

Move the CLI-backed provider adapters and ffmpeg-static from dependencies to optionalDependencies, and make a missing optional dependency degrade with an actionable error instead of a raw ERR_MODULE_NOT_FOUND.

Why

@tangle-network/browser-agent-driver bundles ~300 MB of platform-native binaries that most consumers never use on the default --provider openai path:

dep	pulls	size
ai-sdk-provider-codex-cli	@openai/codex (Codex CLI binary)	~177 MB
ai-sdk-provider-claude-code	@anthropic-ai/claude-agent-sdk (ripgrep ×5 platforms)	~69 MB
ffmpeg-static	static ffmpeg binary (used only by bad showcase)	~49 MB

Slim sandbox runtime images (e.g. agent-thin, which only ever runs --provider openai) want to drop these. Today the only way is a post-install rm -rf in the image build (see agent-dev-container #3369). Declaring them optionalDependencies lets those consumers use the standard npm install --omit=optional instead.

Change

• package.json: the three deps move to optionalDependencies. Default installs are unchanged — optional dependencies still install by default, so the full provider + showcase surface works out of the box.
• Graceful degrade: the five dynamic import() sites for the two providers (brain/model-client.ts, supervisor/critic.ts) now go through loadOptionalModule(), which maps a missing module to a clear message: "The 'codex-cli' provider requires the optional package "…", which is not installed. Reinstall … without --omit=optional … or use --provider openai." This matters because the keyless default resolves to claude-code; on an --omit=optional install a bare bad run <url> now explains itself instead of crashing with a module-resolution stack trace.
• ffmpeg-static already degrades gracefully (lazy require in try/catch → system ffmpeg → skip video), and the postinstall provider-patch step already fs.existsSync-guards the patched file, so --omit=optional never breaks install.

Tests

tests/optional-dependency.test.ts covers loadOptionalModule: resolves a present module, maps ERR_MODULE_NOT_FOUND to the actionable message, and passes unrelated errors through.

Notes

• Non-breaking → patch. No API/CLI surface change; default install footprint is identical.
• Downstream: agent-dev-container's agent-thin image (#3369) can switch its manual provider/ffmpeg rm to npm install --omit=optional once this releases.