docs: add SpaceXAI proof packet

[yangfei222666-9]

Summary

• Add a SpaceXAI proof packet for TaijiOS as an evidence-first AI agent runtime.
• Add a 3-bullet email draft aligned with the proof packet.
• Add a README public-entry link so reviewers can find the packet quickly.

Validation

• markdown sanity check: pass
• Product Spine verifier: partial_product_spine_verified_dirty_tree
• git diff --check: pass
• git diff --cached --check: pass before commit

Boundaries

• Not claiming SpaceX or SpaceXAI endorsement, review, acceptance, or recognition.
• Not claiming production readiness.
• Not claiming real spacecraft, vehicle, robot, satellite, radio, hardware, broker, trade/order, paper-buy, judgment, or promotion capability.
• No secrets, providers, runtime code, or ops_check force-adds are included.

Git scope

This PR contains only:

• README.md
• docs/SPACE_X_AI_PROOF_PACKET.md
• docs/SPACEXAI_3_BULLET_EMAIL_DRAFT.md