fix: keep the shell-exit delta baseline across host rebuilds

[gominimal-aw-bot]

Routing-Key: inbox-route/I_kwDOSUhdos8AAAABMgbl4Q

The shell-exit prompt's "changed since activation" baseline was armed in Host::build, which runs once per host launch rather than once per session. Exiting a session with the "keep filesystem" option tears the host down but leaves the session and its files in place; a later reattach builds a fresh host that re-snapshots the already-modified workspace as its baseline. A second exit then diffs against that dirty baseline, sees no new changes, and drops the save-changes option even though uncommitted changes are still present.

This moves the baseline onto the Session actor, which outlives the host: it is armed once before the first host launches and reused across every teardown and rebuild, so a reattach keeps the activation-time reference point. Host construction now receives the baseline instead of arming its own. A regression test drives a keep-exit followed by a reattach and asserts the pre-exit change is still reported.

Verification

• cargo fmt --all --check --manifest-path target/Cargo.toml — clean
• cargo clippy --workspace --locked --manifest-path target/Cargo.toml -- -D warnings — clean, no warnings
• cargo build --workspace --locked --manifest-path target/Cargo.toml — ok
• cargo test --workspace --locked --manifest-path target/Cargo.toml — all tests passed (exit 0), including the new session::tests::workspace_baseline_survives_keep_exit_and_reattach

Note

Keep the shell-exit workspace change-detection baseline across host rebuilds within a session

• Introduces a WorkspaceBaseline enum in session.rs with Unarmed and Armed variants to track a session-scoped change-detection baseline.
• Arms the baseline once before the first host launch and reuses it on subsequent host rebuilds, so changes made before a shell-exit and reattach are still reported by the SessionDelta RPC.
• Moves baseline arming out of Host::build in session_host.rs; callers now pass an externally-armed Option<Arc<DeltaSource>> instead.
• Adds a regression test workspace_baseline_survives_keep_exit_and_reattach that verifies pre-exit changes remain visible after reattach.

^{Macroscope summarized b1d4c82.}

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 5dbea47f-4427-4f04-ad58-e2b4b064c46e

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

_{Comment @coderabbitai help to get the list of available commands.}