refactor(core): fold single-consumer request modules into model-request

[kitlangton]

What

Folds four session request-shaping modules into model-request.ts, their only remaining consumer. Zero behavior change — every moved function is byte-identical logic under a private name.

These modules existed to be shared across the four hand-rolled request sites (runner, generate, title, compaction). After the envelope unification (#42680, #43555) routed all requests through SessionModelRequest.prepare, that boundary became their sole importer — four files of indirection for ~30 lines each. This makes the code's shape match the architecture that already landed: prepare is the one place requests are shaped, and now it reads top to bottom in one file.

Deleted module	Now
model-headers.ts (SessionModelHeaders.make)	private sessionHeaders
prompt-cache-key.ts (SessionPromptCacheKey.make)	private promptCacheKey
model-hook.ts (SessionModelHook.apply)	private applyModelHooks
model-http.ts (SessionModelHttp.middleware)	private httpMiddleware

How

• packages/core/src/session/model-request.ts — the four functions land beside the file's other helpers, sharing one HookScope type for the identity plugin hooks see. Call sites renamed in place; nothing else touched.
• Verified sole-consumer status before deleting: no other imports of these modules anywhere in src or tests (they were only ever exercised through prepare).

No changeset: internal refactor with no observable change.

Scope

Mechanical fold-in only. No signature, wire, or behavior changes; model-transport.ts keeps its own module (it has service/layer structure, not just a function).

Testing

• bun typecheck (packages/core): clean
• bun run test (packages/core): 1,935 tests, 0 fail