feat(sdk): bind local model for text queries

[limityan]

Summary

• require one process-lifetime model configuration when starting the TypeScript SDK
• install that model into the native SDK Host as a runtime-only configuration and bind all Sessions/Queries to it
• stream assistant text and one terminal Result through the existing Agent Runtime, with bounded cancel/close/process cleanup
• verify the same built SDK through Node and Bun against a real native Host

Type and Areas

Type: Feature

Areas: TypeScript SDK, native SDK Host, Rust core model resolution

Motivation / Impact

Trusted local applications can now start the repository-internal SDK with their own model credentials and execute the minimum end-to-end Agent Query without a preinstalled BitFun application or persisted BitFun model configuration.

The API key is sent once over the managed Host stdin connection, remains process-local, and is removed when the connection shuts down. hostPath is currently required to be an explicit absolute path so PATH or environment-variable resolution cannot redirect the credential-bearing handshake.

This remains a private, unpublished local slice. It does not add callbacks, custom functions, structured output, usage reporting, Session resume, Python support, or native package staging.

Verification

• pnpm --dir sdk/typescript run type-check — passed
• pnpm --dir sdk/typescript test — 63 passed, 1 platform-specific skip
• Node 24.14.1 real native Host smoke — passed; one model request, streamed text, terminal Result, cleanup, and credential scan
• Bun 1.4.0 real native Host smoke using the same dist — passed
• cargo test -p bitfun-sdk-host --tests — 50 passed
• cargo test -p bitfun-sdk-host-app --tests — 21 passed
• focused Core runtime-model overlay, AI client, and Turn-admission tests — passed
• cargo fmt -p bitfun-core -p bitfun-sdk-host -p bitfun-sdk-host-app -- --check — passed
• git diff --check gcwing/main...HEAD — passed

Workspace-wide cargo check --workspace was not completed in this worktree: the first attempt hit a TLS EOF while downloading sherpa-onnx; the retry passed that dependency and stopped because the desktop build expects the separately generated src/mobile-web/dist resource. SDK-scoped checks above are green.

Reviewer Notes

• Protocol version moves from 1 to 2 and remains marked not_delivered; no published compatibility promise exists yet.
• The runtime model overlay is intentionally visible only to Turn admission and AI client construction. Persisted config reads, exports, backups, and disk files remain unchanged.
• Exact diff size: one commit, 29 files, 2000 insertions and 142 deletions (2142 changed lines).
• Rollback is the single commit on this branch.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.