fix: cancel chat runs when deleting conversations (#90)

[aiis2]

Why

Deleting a streaming conversation removed its UI/DB row but left the agent controller and ask_user resolver alive. Deleted runs could continue tools, remain stuck awaiting input, and leave global streaming state with no visible Stop target. A second race allowed a run deleted during message persistence to start later, after its conversation was already gone.

What changed

• Add ID-scoped cancellation that resolves the target ask_user with __ABORT__ and aborts only its controller.
• Reuse that cancellation path for the existing Stop action.
• Atomically remove the deleted ID from conversations and streaming state, recompute the newly active conversation's isStreaming, and clear target-owned question/turn state.
• Preserve the controller entry until the normal finalizer observes aborted and owns cleanup.
• Refuse to start an agent if its conversation was deleted during pre-agent persistence.
• Add product-level regressions for two concurrent streams, deletion during ask_user, and the pre-start deletion race.

Issue

Closes #90

Mainline

Implementation branch created from authoritative origin/master@fdc1cc3 after Spec PR #91 merged. The repository has no origin/main.

TDD evidence

• Initial RED on the unmodified base: 0 pass / 2 fail (target signal stayed live, stale streaming ID remained, ask_user did not settle).
• Review-driven race RED: 2 pass / 1 fail (runReactAgent still started after deletion during persistence).
• GREEN: deletion lifecycle + prior chat settlement tests 5/5 pass.

Verification

• node --test tests/*.test.cjs (162 pass)
• npx tsc --noEmit
• npx tsc -p src/main/tsconfig.json --noEmit
• npx vite build (2346 modules)
• node scripts/smoke-report-preview-isolation.cjs --expect-isolated
• node scripts/smoke-export-origin-isolation.cjs --expect-isolated
• node scripts/smoke-export-runtime-compatibility.cjs --expect-isolated
• production dependency audit: 0 vulnerabilities
• full dependency audit: 0 vulnerabilities
• git diff --check origin/master...HEAD