feat(agent): summarize the conversation when the context window fills

[winetree94]

Why

A session that outgrew its model context window had exactly one escape: the model calling new_context, which discarded the conversation without summarizing it. Nothing acted on the token budget on its own, and a provider that refused an oversized history failed the turn outright.

This ports the local-compaction design from the Codex CLI (codex-rs/core/src/compact.rs). Codex's remote compaction variants depend on OpenAI-only endpoints, so they are out of scope.

What

ConversationCompactor replaces a spent window with a summary of the work so far, keeping past user messages within a token budget so the original asks survive. It runs:

• before a turn that starts on an already-full window,
• after any tool round that crosses the budget,
• on request via /compact (session.compact RPC),
• once as recovery when a provider refuses the history itself.

The replacement history holds user messages only, so it cannot carry a function_call_output whose function_call was dropped — a shape both provider APIs reject. It reuses the context-epoch machinery new_context already established, so the timeline still shows everything; only the model forgets.

Every policy value lives in one CompactionPolicy block: the 0.9 trigger ratio, the 20k retention budget, and both prompts.

Notable decisions

• CompactionTarget instead of a ModelRequest prototype. The between-turns path would otherwise have to rebuild an agent's system prompt. The compactor now owns its own instructions, so the automatic and requested paths use exactly the same prompt.
• Attachments are dropped on compaction. The retention budget can only measure text; carrying images and files over would leave the window as full as before, defeating the point. What they showed belongs in the summary.
• A guard Codex explicitly waives. If a compaction fails to shrink usage, compacting again cannot help either, so the runner stops rather than buying one summary per round for the rest of a long turn.

Verification

• dart run melos verify — all 12 gates pass
• dart run melos verify:debug — all 6 Linux E2E shards pass (real Debug Flutter runner + embedded daemon)
• New tests: 17 unit, 1 contract, 1 real-daemon vertical slice (asserts the compacted window round-trips through the daemon), 2 widget. Registered as the context.compaction feature contract.
• No golden changes: the two context-window dividers were factored onto one shared widget, so pixels are unchanged.