fix(interrupt): always surface a user-visible stop reason

[ophirhan]

Summary

Interrupted turns that ended with empty final text (hard stop, tool timeout, client-gone, lease takeover) could settle as a blank chat: no reason line, error_retained=False, and Desktop skipped the no-reply card for interrupted turns.

This PR always produces a user-visible stop reason on empty interrupts and carries it through host → gateway/API/TUI → Desktop.

Changes

• Agent: format_interrupt_user_text / ensure_interrupt_final_response — never emit bare Operation interrupted.; prefer tool_reason, exit reason, or session-id pointer into agent.log
• Finalizer: fill reasoned final_response (and error) before persist on interrupted turns
• TUI gateway: interrupted message.complete always sets error + non-empty text; system lifecycle stops pass distinct tool_reason (not mislabeled user Stop)
• Gateway chat + API runs: deliver interrupt reason instead of discarding bare cancel
• Desktop/WebUI: interrupted empty settle paints a non-retryable stop notice; message.complete falls back to error when text is empty

Related

• Upstream issue: [Bug]: Interrupted turn (exit 130) delivers zero user feedback — silent death until user re-pings; no distinction between deliberate stop and client disconnect #84207
• Related PR: fix(agent): visible closing message and structured stop_kind for interrupted turns #84236
• Relay client companion (already open): fix(android): show interrupt reason instead of bare Response/Run interrupted Codename-11/hermes-relay#634

Test plan

• ./scripts/run_tests.sh tests/agent/test_format_interrupt_user_text.py
• ./scripts/run_tests.sh tests/agent/test_close_interrupted_tool_sequence.py
• TUI interrupt suite (test_interrupt_dead_compute_host, hook, background review, does_not_wait)
• Desktop vitest session-states-interrupted-settle.test.ts (CI)
• Manual: stop mid-tool / terminal batch timeout → chat shows Operation interrupted: … not blank

[ophirhan]

Rebased onto current main (84f748ce65f).

• Desktop no-reply helpers now live in session-no-reply.ts on main — moved the interrupted-empty notice there and wired markTurnWithoutReply to paint a non-retryable stop card instead of going blank.
• Python unit tests: tests/agent/test_format_interrupt_user_text.py 7/7 green.
• Desktop: session-states-interrupted-settle.test.ts 2/2 green.

Related: #84207 · pairs with Codename-11/hermes-relay#634 (phone display).

[ophirhan]

Rebased onto current main (a23223660ab). Unique commit is still the interrupt-reason fill only. Ready for review.