Improve error messages for common CLI mistakes

[Dave93]

Summary

When users make common mistakes (wrong process name, daemon not running, etc.), Velos should give clear, actionable error messages instead of generic ones.

Examples to improve

Situation	Current output	Expected output
velos stop unknown-app	Error: process not found	Error: no process named 'unknown-app'. Run velos list to see running processes.
velos logs myapp when daemon is down	generic error	Error: Velos daemon is not running. Start it with velos start .
velos start app.js (file doesn't exist)	generic error	Error: file 'app.js' not found in current directory.

Relevant files

• crates/velos-cli/src/commands/ — command handlers
• crates/velos-client/src/ — IPC client error types

Acceptance criteria

• At least 3 error messages improved with actionable hints
• Error messages are consistent in style (lowercase, no trailing period)
• No new dependencies added

Good first Rust issue — requires only reading existing command code and improving eprintln! / error returns.