feat(mtmharness): add loopback profile updater

[codeh007]

Summary

• Add a dedicated loopback-only /mtm-update RPC for stable-version checks and explicit pnpm update mtmharness --latest.
• Resolve the active profile from DSH ctx.baseUrl, validate the profile composition, verify the installed manifest, serialize updates, and report restart-required without exiting the host.
• Add loopback-gated check/update controls to the MTM Coding settings card; standalone/embed clients remain unchanged.

Verification

• pnpm --filter mtmharness run check
• pnpm --filter mtmharness run pack:check
• Focused updater, client state, loopback gating, cancellation, and legacy artifact tests pass.
• Browser artifact purity check passes.

Release gate

• mtmharness-v0.6.0 is not created yet.
• The release workflow also requires mtmcanvas@0.2.0, which is not published.
• Local npm whoami is unauthenticated; no publish was attempted.

Addresses codeh007/gomtm#681

[coderabbitai]

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