Enhance user-facing error messages for remote server install failures

[alokedesai]

Description

Enhance user-facing error messages in transport.rs for remote server install failures. Instead of showing the generic "Script exited with code N: " for all ScriptFailed errors, the user_facing_error() method now detects specific patterns in stderr and produces targeted, actionable messages:

Failure condition	Old message	New message
Permission denied / Read-only FS	Script exited with code 1: mkdir: cannot create directory...	Cannot create install directory — check write permissions on your home directory (exit code 1)
No space left on device	Script exited with code 2: No space left on device	Not enough disk space on the remote host — free up space and try again (exit code 2)
SSH disconnect (exit 255, empty stderr)	Script exited with code 255:	SSH connection was lost during installation — please reconnect and try again
Unsupported architecture	Script exited with code 2: unsupported arch: armv7l	This remote host uses a armv7l processor, which is not supported. Warp SSH extension requires x86_64 or aarch64 (arm64).
Other errors	(unchanged)	(unchanged — still uses truncated stderr format)

Linked Issue

N/A — part of remote server install failure improvements initiative.

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Testing

Added 12 unit tests in transport_tests.rs covering all error patterns and edge cases:

• Permission denied, read-only filesystem, disk full, SSH disconnect, unsupported arch
• Default format fallback for unrecognised errors
• Long stderr truncation
• Whitespace-only stderr with exit code 255
• Exit code 255 with real stderr content (should NOT trigger SSH disconnect)
• Body text varies by setup stage

All tests pass. cargo fmt and cargo clippy pass clean.

• I have manually tested my changes locally with ./script/run

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Conversation: https://staging.warp.dev/conversation/cd4e30c2-95e7-4516-860d-427a6afbc70f
Run: https://oz.staging.warp.dev/runs/019e09f4-0711-7661-aa65-13b07ae4ddcb

This PR was generated with Oz.