fix(agent): make the models-barrel import in turn-context type-only

[w0nche0l]

Summary

• turn-context.ts runtime-imported the entire @openrouter/sdk/models namespace to read a single value: EasyInputMessageRoleUser.User, i.e. the string 'user'. Because turn-context sits on the static import path of @openrouter/agent/tool (tool → agent-tool → conversation-state → turn-context, new in 0.9.0), every consumer that statically imports the tool subpath now evaluates the Speakeasy models barrel — hundreds of modules of top-level Zod schema construction — at module load.
• On Cloudflare Workers this added ~200ms of startup CPU per worker and pushed OpenRouter's large workers past the 1s script-validation ceiling (error 10021), forcing openrouter-web#33740 to revert the 0.9.0 adoption. Independent measurement: evaluating the three hot subpaths in a bare V8 isolate went from 0.007ms (0.7.2) to ~233ms (0.9.0), and back to ~8ms with this import removed. openrouter-web is re-landing 0.9.0 with this same fix as a bun patch in openrouter-web#33860 until it ships here.
• Fix: make the import type-only (erased at compile time) and inline the 'user' literal — behavior identical, still typechecked against models.EasyInputMessage.

Regression guard

New unit test tests/unit/startup-import-closure.test.ts walks the static runtime import graph of the startup-hot subpaths (/tool, /tool-types, /stop-conditions) from source and fails if any of them reaches @openrouter/sdk at runtime. Type-only imports and dynamic import() are exempt by construction. Verified the test fails when the import is made runtime again.

Test plan

• pnpm test — 97 files, 1156 tests, no type errors, coverage thresholds met
• pnpm typecheck, pnpm lint, pnpm build green
• Negative test: reverting the import to runtime makes the new guard fail with the full offending chain

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