feat(agent): display model name at end of completed chat session

[Aarjav Jain (A2rjav)]

Summary

Closes #883

Adds a small "Session completed with " footer that renders beneath the conversation once an assistant turn finishes. Users had no in-UI signal of which provider/model handled a given session, which made comparing model behaviour and debugging unexpected output harder than necessary.

Changes

New component: apps/agent/entrypoints/sidepanel/index/SessionCompletedFooter.tsx

• Pure presentational component, no side effects
• Subtle muted styling that doesn't compete with the conversation
• formatSessionCompletedLabel helper resolves the best label:
  • LLM targets → Provider.name (the user's configured display name)
  • ACP targets → <adapterName> · <modelLabel> (e.g. Codex · gpt-4-turbo)
  • Graceful fallbacks when ACP metadata is partial (adapterName-only, modelLabel-only, or Provider.name)
• Returns null when the provider is undefined or no label can be resolved

Wire-up: apps/agent/entrypoints/sidepanel/index/Chat.tsx

• Renders the footer inside the chat <main> after the error banners
• Visibility gated on: messages.length > 0 && status === 'ready' && !chatError && !agentUrlError
• Hidden while streaming, while restoring a conversation, and while an error is showing

Tests: apps/agent/entrypoints/sidepanel/index/SessionCompletedFooter.test.ts

• 6 cases covering: undefined provider, LLM target, full ACP target, ACP with only adapterName, ACP with only modelLabel, ACP with neither

Test plan

• bun test apps/agent/entrypoints/sidepanel/index/SessionCompletedFooter.test.ts — 6/6 pass
• bun test apps/agent/entrypoints/sidepanel/index/ — 18/18 pass (no regressions in adjacent files)
• bunx @biomejs/biome check on the three touched files — clean
• Open the sidepanel, send a message, wait for response → footer appears under the assistant reply with the active provider name
• Switch to an ACP agent, send a message → footer shows <adapter> · <model>
• Trigger an error mid-stream (e.g. invalid auth) → footer is hidden while the error banner is shown
• Start a new conversation → footer is hidden until the first reply lands

Visual

┌──────────────────────────────────────────┐
│  user: summarise this page               │
│                                          │
│  assistant: ...long answer here...       │
│                                          │
│       Session completed with OpenAI GPT-4 │
└──────────────────────────────────────────┘

[greptile-apps]

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