feat(go): composer + attachment UX overhaul

[baryhuang]

Summary

Rebuild of the OpenAgents Go composer (macOS + iOS) plus a top-to-bottom attachment-pipeline overhaul. Cleared a stack of dogfooding issues in one cohesive change. v0.2.1 → 0.2.2.

Composer

• Replace the SwiftUI TextEditor with a platform-native NSTextView / UITextView representable so we own the responder.
• IME-safe Return-to-send: gates on hasMarkedText() (macOS) / markedTextRange (iOS) so pressing Enter mid-Pinyin or mid-Kana confirms the candidate instead of false-sending.
• macOS image paste: extend readablePasteboardTypes + acceptableDragTypes so Cmd+V actually validates for image-only pasteboards (the system bonk before this change came from menu-validation, not from our paste(_:) override). Override handles PNG → JPEG → TIFF → NSImage fallback, plus file URLs from Finder.
• iOS image paste: subclass UITextView; canPerformAction + paste(_:) read UIPasteboard.images. Hardware Cmd+V works; soft Return inserts newline to match iMessage.

Bubble layout

• MessageBubble drops maxWidth: .infinity on the prose-only path so short user messages hug their text. Code / table content still claims the full row.

Toolbar titles

• macOS: detail column's navigationTitle set to the current thread title (replaces the default "OpenAgents Go" in the window toolbar). Subtitle shows participating agents.
• ThreadListView gets a visible workspace name + slug header at the top of the sidebar on macOS (NavigationSplitView doesn't surface a sidebar .navigationTitle anywhere visible).

Stop button

• New WorkspaceAPI.sendAgentControl mirrors the React app.
• WorkspaceStore.stopAllAgents fans out workspace.agent.control events with optimistic "Stopping…" status, retries once after 3s, clears the flag on terminal status from the message poll.
• ChatGPT-style: send-arrow flips to a red stop circle while an agent is working; grays out while a stop is in flight.

Attachment UX

• Dedup: optimistic message content is rewritten in-place to finalContent after api.sendMessage returns so the existing poll dedup matches. Fallback matcher compares the set of attached filenames for the mid-upload race. Fixes the "two bubbles per upload" symptom.
• Auto-downsample to ≤ 2000px: new ImageDownsampler (ImageIO-based) wired into every ingest site (paste, photo picker, file import). Targets the exact Anthropic many-image-request limit so a Retina screenshot can't poison the conversation downstream.
• Thumbnail chip: image attachments render as a 48×48 thumbnail with X overlay (decoded via ImageIO, cached). Non-image keeps the Slack-style filename + size row.
• Long-text paste → file: paste ≥ 5,000 chars converts to a Pasted-text-<ts>.txt attachment instead of dumping into the input. Threshold matches ChatGPT's published behavior.

Test plan

• Builds clean for OpenAgentsGo_macOS and OpenAgentsGo_iOS (Debug + Release).
• macOS: short message hugs the bubble; long paragraphs wrap at row width; code blocks still claim full row.
• macOS: Pinyin → space → Enter confirms candidate; second Enter sends. No mid-composition false sends.
• macOS: Cmd+V on a screenshot adds an attachment chip (no bonk); pasted Retina screenshot is downsampled to ≤ 2000px before upload; downstream agent's Read no longer trips Anthropic's 2000px limit.
• macOS: long-text paste (≥ 5000 chars) creates Pasted-text-…txt chip; the text view stays empty.
• macOS: single bubble per send (no duplicate "uploading…" + final pair); chip remove (X) clears the pending attachment.
• macOS: Stop button appears while an agent is working; tapping sends a stop control event; flips back to send arrow on terminal status.
• macOS: window toolbar shows the thread title + agent names; sidebar shows workspace name.
• iOS regression sweep on simulator (paste, photo picker, IME, Stop) — not run in this PR.

Files

New:

• packages/go/OpenAgents/Views/ComposerTextView.swift
• packages/go/OpenAgents/Helpers/ImageDownsampler.swift

Modified:

• packages/go/OpenAgents/Views/ChatView.swift — bubble layout, composer wiring, send/stop button, chip redesign, downsample at ingest sites.
• packages/go/OpenAgents/Views/ThreadListView.swift — workspace header.
• packages/go/OpenAgents/State/WorkspaceStore.swift — stop state + dedup.
• packages/go/OpenAgents/Networking/WorkspaceAPI.swift — sendAgentControl.
• packages/go/OpenAgents/Models/Attachment.swift — makeThumbnail.
• packages/go/OpenAgents/Models/Message.swift — localStoppingStatus helper.
• packages/go/project.yml + regenerated project.pbxproj — version bump.

Deferred

• Sent-bubble attachment cards (render [file.png](url) lines as proper thumbnail cards inside the message bubble) — explicitly punted to keep the diff bounded; markdown link rendering is acceptable now that dedup makes it a single line.

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[baryhuang]

Ready to review. This is package version 0.2.2

[zomux]

Excellent overhaul of the composer and attachment pipeline. Good use of platform-native text views for IME-safe input, proper image downsampling, and clean separation of concerns. Minor nit: iOS force-unwrap in insertHardwareNewline (textRange(from: endOfDocument, to: endOfDocument)!) — low risk but could be guarded.