Add official DeepSeek Harness support

[Yaxin9Luo]

What changed

• adds a first-class adapter for the official DeepSeek Harness released headless profile
• supports DeepSeek across poster, deck, landing page, video, Canvas editing, Web Settings, CLI, and harness matrix flows
• probes installed DSH capabilities, rejects the old preview CLI with upgrade guidance, and uses an invocation-local model overlay
• keeps DeepSeek credentials isolated from AutoDesign helper-model provider credentials

User impact

Users can select DeepSeek Harness as their coding agent, optionally choose DeepSeek V4 Flash or Pro, and use the same generation and editing surfaces as the existing harnesses.

Validation

• 41 focused unit and contract tests passed
• full autodesign smoke passed all 53 sections
• frontend typecheck and production build passed
• Python compile and git diff check passed
• official npm latest and installed DSH both verified as 0.1.0-rc.6
• real DSH coding-agent smoke passed against the public checkout

Per the requested rollout order, a real artifact E2E run from the merged public main follows this PR merge.