refactor: separate loader defaults, bootstrap runtime opts, narrow chatserver contract

[dgageot]

teamloader/defaults.Opts always registered global runtime factories (jscommands.Register(), runtime.RegisterHarness) as a side effect of asking for loader defaults, which made it impossible to build a loader's options without also mutating process-wide runtime state. The model-switcher and budget wiring that turns a teamloader.LoadResult into runtime options was also duplicated between the CLI's initial run and its spawned sub-runtimes, and chatserver depended on the full runtime.Runtime interface even though it only ever drives three methods of it.

This splits defaults.Opts() into a registration-free defaults.LoaderOpts() plus the existing Opts(), which now just calls LoaderOpts() after doing its registration as before — so nothing changes for existing callers. It adds a pkg/bootstrap package whose RuntimeOpts(loadResult, runConfig) assembles the ModelSwitcherConfig, shared models.dev store, and budget options from a LoadResult; cmd/root/run.go now calls it from both the initial and spawned runtime paths instead of duplicating the assembly. Finally, chatserver defines a private runtimeRunner interface with just RunStream, Resume, and ResumeElicitation, and the agent loop, server handlers, and runtime pool are narrowed to that contract instead of the full runtime.Runtime.

Public interfaces, HTTP schemas, and serving budget behavior are unchanged; this is a pure internal reshuffle with new regression tests for bootstrap, defaults.LoaderOpts, and the narrowed chatserver contract (including a minimal fake runtime). Docs (docs/guides/go-sdk/index.md) and the dependency guard test (e2e/dependencies_test.go) were updated to reflect the new pkg/bootstrap package; no go.mod/go.sum changes were needed.