fix: restore quickstart page, accept AAO API keys on test agent

[bokelley]

Summary

• Restore docs/quickstart.mdx (deleted in Restructure docs nav, introduce Sage as protocol identity #1509 nav restructure) with wire-format examples verified against the live training agent
• Add WorkOS API key validation to training agent so AAO dashboard keys (sk_/wos_api_key_ prefix) work as bearer tokens
• Remove hardcoded PUBLIC_TEST_AGENT_TOKEN fallback from source (env-var only)
• Strip exposed p6IQ token from all current docs, replace with $ADCP_AUTH_TOKEN env var references

Closes #1576

Test plan

• Stood up local server (postgres + training agent), tested all 4 quickstart curl commands
• Verified get_products response shape including pricing_option_id
• Verified create_media_buy with correct account, budget (number), start_time, end_time
• Verified error response (isError: true with code/message)
• Verified auth rejection with wrong token returns clear error message
• TypeScript compiles clean, 192 training agent tests pass
• Pre-commit hooks pass (all test suites, docs nav validation, schema validation)
• Pre-push hooks pass (Mintlify broken link check, version sync, accessibility)

Follow-ups

• Rotate PUBLIC_TEST_AGENT_TOKEN env var in production (the p6IQ value is compromised)
• dist/ versioned snapshots still contain the old token (build artifacts, not source of truth)

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