docs: point the T12 belt at the engine heartbeat and the CLI

[DealAppSeo]

T12 cannot live in this repo. The belt points at the engine heartbeat GET /health on the default engine origin in src/cli/status.ts, and at the CLI only: trustshell status and trustshell proof <agentIdOrSlug> --verify.

No worker. No secret print. No HeyGen. No live stake.

Related to #191.

[vercel]

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Project	Deployment	Actions	Updated
trustshell-landing	Ready	Preview	Sep 26, 2026 10:46pm UTC

[strix-security]

Strix Security Review

No security issues found.

Review summary

The PR adds only documentation (docs/T12_BELT.md) and a test (tests/t12-belt.test.ts). It changes no runtime code. The documentation references the engine origin https://repid-engine-production.up.railway.app, which is a public Railway endpoint already defined as DEFAULT_ENGINE in the unchanged src/cli/status.ts, not a secret or credential. The test reads fixed local file paths and performs static string/regex assertions, with no user-controlled input or dangerous sink. No security vulnerabilities were identified in the changed code.

_{Updated for 850642c.}

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[strix-security]

The PR adds only documentation (docs/T12_BELT.md) and a test (tests/t12-belt.test.ts). It changes no runtime code. The documentation references the engine origin https://repid-engine-production.up.railway.app, which is a public Railway endpoint already defined as DEFAULT_ENGINE in the unchanged src/cli/status.ts, not a secret or credential. The test reads fixed local file paths and performs static string/regex assertions, with no user-controlled input or dangerous sink. No security vulnerabilities were identified in the changed code.

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Reviewed by Strix
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