refactor(core): route title and compaction through shared model requests

[kitlangton]

What

Routes the two remaining hand-rolled session-scoped LLM request envelopes — title generation and compaction summaries — through the shared SessionModelRequest.prepare boundary established in #42680. Callers keep policy (transcript selection, streaming, usage recording, event publishing); prepare owns all provider-visible shaping.

Before / After

session/title.ts (title generation)

Aspect	Before	After
Model	title agent's model (or session model fallback) via SessionRunnerModel.resolve	unchanged — caller-resolved model passed to prepare scope
Session headers	hand-rolled SessionModelHeaders.make	unchanged, via prepare
Prompt cache key	none	fork-aware session cache key (same prefix identity as the session's main requests)
model.request hooks	hand-rolled SessionModelHook.apply	unchanged, via prepare
Session context hooks	not applied	applied, with an empty tool snapshot
Media filtering / image bounds	none	applied (no-op today: title requests are text-only)
Tools	tools: []	empty Tool.Snapshot → still [] on the wire
Transport	always HTTP with hook middleware	HTTP only (webSocket never passed); hook middleware attached only when HTTP hooks are registered, matching the runner

session/compaction.ts (compaction summary)

Aspect	Before	After
Model	runner/manual-resolved model passed as separate model/ref/cost fields	one SessionRunnerModel.Resolved carried through AutoInput/Plan
Session headers	hand-rolled	unchanged, via prepare
Prompt cache key	SessionPromptCacheKey.make(session.id) — a forked session used its own id	fork-aware: forked sessions reuse the fork root's cache key, matching the runner
model.request hooks	already applied (bolted on by cb39ea1 — same drift pattern generate-node.ts had before #42680)	unchanged, now uniform via prepare
Session context hooks	not applied	applied, with an empty tool snapshot
Media filtering / image bounds	none	applied (no-op today: the summary prompt is serialized text)
Tools	tools: []	empty Tool.Snapshot → still [] on the wire
Transport	always HTTP with hook middleware	HTTP only (webSocket never passed); hook middleware attached only when HTTP hooks are registered

Audited and left alone

• core/src/generate.ts (Generate.text) — a generic, non-session generate utility. It has no session scope: no session ID, no agent, no session headers, cache key, or hooks apply. Putting it behind a session-scoped boundary would invent scope it does not have, so it stays on LLMClient.generate directly.
• The webSocket: "session" opt-in remains exclusive to the durable runner; neither title nor compaction passes it.

Session context hooks stay away from title and compaction requests.
Unification must not change observable plugin behavior: title and compaction requests never ran session.context hooks before, so they still do not. prepare gains a contextHooks: false opt-out — context hooks shape the agent conversation, and title/compaction requests are not part of it, so their transcripts pass through unchanged — and both behaviors are pinned by tests ("keeps session context hooks away from title/compaction requests"). Conversation requests (runner, session.generate) keep running them. scope.tools also became optional: title and compaction requests carry no tools, and the empty snapshot now lives at the boundary that required it instead of duplicated at each call site.

How

• SessionTitle and SessionCompaction now depend on SessionModelRequest.Service and build their requests with prepare({ scope, transcript }), dropping their private uses of SessionModelHeaders, SessionModelHook, SessionModelHttp, SessionPromptCacheKey, PluginHooks, and App.
• Each passes an empty Tool.Snapshot; Prepared.executeTool is unused since neither site executes tools.
• SessionCompaction.AutoInput/Plan collapse model/ref/cost into one SessionRunnerModel.Resolved; the runner's compactionInput passes loaded.model directly.
• No fields were added to Prepared; policy stays in the callers.

Scope

• packages/core/src/session/title.ts
• packages/core/src/session/compaction.ts
• packages/core/src/session/runner/llm.ts (compaction input shape only)
• packages/core/test/session-title.test.ts, test/session-compaction.test.ts, test/config/compaction.test.ts (fixture shape + new coverage)
• .changeset/unify-request-envelope.md

Testing

• New: title requests assert the session prompt cache key, empty tool list, and system prompt; a registered session.context hook now reaches title requests; forked-session compaction asserts the fork root prompt cache key.
• Existing manual-compaction test still pins headers, cache key, and durable event order.
• bun typecheck and bun run test from packages/core: 1891 pass, 0 fail.