refactor: split the agent turn loop and the TUI app into modules

[hakula139]

Summary

As the first part of #148, this splits the two largest files by concern without changing behavior. The slash-command move into a core command module will follow in a separate PR.

• Engine turn modules. agent.rs moves under the engine. The turn loop and stream collection move to engine/turn.rs, while the tool round, its permission gate, and approvals move to engine/dispatch.rs. The auto-compaction trigger moves to engine/compaction.rs with the summarization request. TurnAbort and await_unless_aborted are shared by all three modules, so they move to engine/abort.rs to keep the dispatch module from importing the turn module that calls it. Since the moved items are used only inside the engine, their visibility narrows to pub(super).
• Post-compact message in the session. SUMMARY_PREFIX and the helpers that write and strip the synthetic post-compact message move to session/post_compact.rs. On resume, session history strips the prefix. This move also removes its import from the engine layer above it.
• App modules. tui/app.rs retains the App state and its event loop. Terminal input, along with the actions and ops it produces, moves to app/dispatch.rs. Engine and git probe events, together with the turn lifecycle they drive, move to app/events.rs. Frame drawing moves to app/render.rs. Methods called across these modules have their visibility widened to pub(super).
• Tests beside their code. Each test moves alongside the function it covers. The turn fixtures shared by the turn, dispatch, and compaction tests join the engine fixtures in engine/testing.rs. The app fixtures live in tui/app/testing.rs, and the app's render snapshots move with their test module. Two existing tests change: the approval-event check now asserts the exact event list, and the duplicate in-test writer is replaced with the shared crossterm_test_terminal fixture.
• Docs. The module map drops the Agent layer, and the design-doc Sources point to the new files.

Part of #148.

Test plan

• cargo fmt --all --check, cargo clippy --all-targets -- -D warnings, and cargo test pass. The test count matches main, with one new test.
• The new test, modal_gate_lets_mouse_scrolls_reach_chat, verifies that the modal gate passes mouse scrolls through to chat. It fails if the gate swallows them.
• pnpm format, pnpm lint, and pnpm spellcheck pass.
• Codecov counts code in the new files as added lines, so moved lines that lacked coverage on main appear as patch misses. Most are test assertion arms that run only when a test fails: the overflow-retry check in the compaction tests, the reply_of fixture's panic, and the text-block check in the post-compact tests. The test_app fixture's browser stub runs only if a test opens a browser. The jump overlay's guard for a pill wider than the chat area cannot trigger because the label is truncated to fit.

[codecov]

Codecov Report

❌ Patch coverage is 99.85391% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/oxide-code/src/engine/compaction.rs	99.65%	2 Missing ⚠️
crates/oxide-code/src/engine/testing.rs	99.23%	1 Missing ⚠️
crates/oxide-code/src/session/post_compact.rs	98.24%	1 Missing ⚠️
crates/oxide-code/src/tui/app/render.rs	99.74%	1 Missing ⚠️
crates/oxide-code/src/tui/app/testing.rs	99.22%	1 Missing ⚠️

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