fix(desktop): keep interim assistant narration on message.complete

[giggling-ginger]

What does this PR do?

On Desktop, a multi-step turn (text → tool → text → tool → …) streams mid-turn
narration live, then collapses to only the final message the instant
message.complete lands. Tool activity and intermediate assistant text flash
for a split second and then disappear — exactly the symptom reported in #61822.

Root cause: completeAssistantMessage used an unconditional replaceTextPart
that dropped every text part and re-appended only
message.complete.text (the agent's last assistant segment). Desktop's
live-completion path was the sole outlier: messaging gateway, Ink TUI, and
Desktop's own reload path already keep interim text.

This wires Desktop to honor the existing display.interim_assistant_messages
setting (default true) and merges final text without discarding earlier
narration.

Related Issue

Fixes #61822. Also Fixes #54905 and #61297 (same Desktop root cause).

Salvaged from concurrent PR #61447 by @lucasfdale (authorship preserved via
cherry-pick). Closed auto-PR #61866 was empty/broken and not used.

Type of Change

• 🐛 Bug fix

Changes Made

• apps/desktop/src/lib/chat-messages.ts — pure mergeFinalAssistantText(parts, finalText, keepInterim)
• apps/desktop/src/app/session/hooks/use-message-stream/index.ts — use merge + config atom instead of drop-all-text
• apps/desktop/src/store/session.ts — $keepInterimAssistantMessages atom (default true)
• apps/desktop/src/app/session/hooks/use-hermes-config.ts — set atom from config.display.interim_assistant_messages
• Tests: unit + integration (37 passing)
• Comment-only clarifications in cli-config.yaml.example + hermes_cli/config.py

How to Test

1. Desktop: multi-step prompt that narrates between tools
2. Before: bubble collapses to final line only
3. After (default): interim narration + tools + final answer remain
4. display.interim_assistant_messages: false restores lean collapse
5. Automated:

   cd apps/desktop && ../../node_modules/.bin/vitest run --config vite.config.ts --environment jsdom \
     src/lib/chat-messages.test.ts \
     src/app/session/hooks/use-message-stream/interim-narration.test.tsx

   (37 passed)

Checklist

• Conventional Commits
• Searched concurrent PRs (fix(desktop): honor display.interim_assistant_messages — stop collapsing mid-turn narration #61447 salvage; fix(61822): [Bug]: Tool calls and intermediate tool execution text not showing up in #61866 closed empty; feat(cli): register interim_assistant_callback to show mid-turn agent messages #61880 is classic-CLI-only complementary)
• Tests added and passing
• Contributor credit preserved (cherry-pick author + AUTHOR_MAP)

[alt-glitch]

This was generated by AI during triage.

Related: salvage of concurrent OPEN PR #61447 (@lucasfdale, authorship preserved via cherry-pick — same completeAssistantMessage drop-all-text root cause; salvage, not duplicate). Adjacent interim-narration cluster: #59831 / #60453 (Codex commentary as interim prose) and #53350 / #53369 (Desktop completion reconcile). Maintainer to pick between this and #61447.

[giggling-ginger]

Closing in favor of the original fix PR #61447.

#61997 was only a salvage cherry-pick of @lucasfdale's work onto current main to attach #61822. No independent code changes beyond AUTHOR_MAP. Prefer merging #61447 directly and linking the issue there rather than duplicating the PR surface.

See review note on #61447.