refactor: release replaced runtimes on switchModel; name service constructors

[K-Mistele]

Summary

Follows a review of fold-core and fold-agent against the make / layer pattern from the RcMap post. It fixes one bug, removes two internal provides, and gives each service constructor a name.

Bug: switchModel kept replaced runtimes open (fold-core)

Root runtimes were built into the session scope, so every model switch left the old runtime's provider HTTP client and layers open until the session closed. A failed switch also left the new runtime open.

Now each root runtime gets its own child of the session scope (OpenSession.ts, provisionRootRuntime):

• After a successful switch, the old runtime's scope closes. It's idle at that point, because root runs hold the same gate.
• After a failed switch, the new runtime's scope closes.

New test: SwitchModel.vi.test.ts › switchModel releases the replaced runtime…. It fails on main (expected [] to deeply equal ['first']) and passes here.

Fewer internal provides (fold-agent)

• SessionTitleGenerator service. It builds the title model once per session. Before, every title provided a fresh LanguageModel layer. The service is provided per session in Launch.ts, next to the output store and the JSONL log, and withGeneratedTitles yields it.
• No Effect.context capture for titles. withGeneratedTitles used to capture FileSystem only to refresh the summary index after each title. listSessionSummaries already rebuilds any record whose log changed, and the title append changes the log, so that refresh and refreshSessionSummaryIndex are gone. The index now updates on the next listing, which the TUI runs after every durable change. The only Effect.context left in fold-core and fold-agent is Session.open's capture of the tools' generic R, whose services can't be known ahead of time.
• readJsonlEntries(path). It yields FileSystem and reads the log. loadSessionSummary now uses it instead of building a whole JSONL EventLog layer (Ref, PubSub, Semaphore) just to read one file.

Cleanups

• Removed the SkillSource service key (fold-core). Nothing provided or yielded it. The SkillSourceService type stays.
• make inlined into every service class, following the post. Each service this PR touches is declared as class X extends Context.Service<X, Shape>()('id', { make: Effect.gen(...) }) with static readonly layer = Layer.effect(this, this.make) (or (args) => Layer.effect(this, this.make(args)) for services that take config). No separate makeX effects or liveXLayer constants remain.
  • fold-core: the ToolRuntime, AgentRuntime, Session, HookRunner, and AgentEvents implementations move into their *Service.ts modules, and the *Layer.ts files are deleted. ToolEventSink, SessionControls, and Profiles follow the same pattern. Callers now use X.layer (for example liveToolRuntimeLayer becomes ToolRuntime.layer and layerHookRunner(hooks) becomes HookRunner.layer(hooks)).
  • fold-agent: Photon, ManagedBinaries, OutputStore, and SessionTitleGenerator. layerOutputStore(opts) becomes OutputStore.layer(opts).
  • Moving the implementations re-indents large blocks, so review with whitespace hidden (git diff -w).

Left out on purpose

• layerCodingToolServices and layerJsonlNode stay. They're documented in the README as helpers a host calls in its own startup code, which is where wiring belongs.
• Launch.ts keeps Layer.build + Effect.provideContext(sessionServices). That provides a built layer; it doesn't capture the environment. Effect.provide(layer) would close the session's services when launchSession returns, but they must live as long as the session, as the README documents.
• fold-cli is untouched.

Verification

• bun run lint, bun run format:check, and bun run typecheck all pass.
• bun run test passes everywhere except 2 live tests in fold-agent/test/Codex/CodexLive.vi.test.ts. These run against the real Codex backend with local ~/.fold/auth.json credentials, which reject the model ('gpt-5.5' model is not supported when using Codex with a ChatGPT account). They're skipped in CI and unrelated to this change.

🤖 Generated with Claude Code

[K-Mistele]

Please yield this instead of using context.