Improve clipboard image pasting and upload performance over SSH

[avafloww]

Clipboard screenshots commonly arrive as TIFF, which remote coding agents may not accept. This improves the existing “Upload and Paste Path” action by converting TIFF/other bitmap formats to PNG, preserving supported compressed formats, and giving pasted images unique filenames. Uploads resolve the SSH Integration conductor through tmux control-mode gateways and can use its home directory before shell integration reports a working directory, including inside ordinary tmux. Successful uploads paste the actual destination after collision renaming; cancelled or obsolete completions cannot paste a path into a different session/connection.

SSH Integration uploads now send sequential 16 KiB chunks instead of 1 KiB. A 490 KiB image requires 31 append requests instead of 490. The included benchmark runs the real remote framer over an SSH PTY: measured upload time fell from approximately 4.2 seconds to 1.4–1.7 seconds, with SHA-256 verification. Larger 64 KiB chunks took approximately 3 seconds because of parser overhead. Cancellation now cleans up temporary files, handles cancellation during the final rename, and avoids leaving finished transfers in “Cancelling…”.

Validation:

• Development build succeeded locally.
• ConductorUploadTests: 5 passed, covering chunk boundaries, byte preservation, progress, cancellation, and append errors.
• NonTextPasteImageTests: 3 passed; NSStringQuotedStringForPasteTests: 22 passed.
• Clipboard image upload/paste manually confirmed over SSH Integration with ordinary tmux. The tmux control-mode gateway path has not been manually exercised.
• python3 tests/benchmark_conductor_upload.py <host> verified synthetic uploads across 1, 4, 8, 16, 32, and 64 KiB chunk sizes.
• git diff --check passed.

Local build/test validation used separate Xcode 27 compatibility changes and rebuilt dependencies. Those changes, generated binaries, and local fish/it2ssh setup are excluded from this PR; the isolated PR branch has not been built independently with an older toolchain. Release notes and the new localized error string are included.