feat(core): add port flag to dev and start

[fpasquet]

Fixes #417

Add port flag to dev and start.

Example usage:

skybridge dev --port 4000
skybridge start --port 4000

or

skybridge dev -p 4000
skybridge start -p 4000

Greptile Overview

Greptile Summary

This PR adds a --port (-p) flag to both skybridge dev and skybridge start commands, allowing users to specify a custom port instead of the hardcoded default of 3000.

Key changes:

• Created resolve-port.ts utility with port validation and resolution logic (flag → env → fallback)
• Updated dev and start commands to accept --port flag and pass resolved port via PORT environment variable
• Updated server code (server.ts, widgetsDevServer.ts, template index.ts, devtools index.ts) to read port from process.env.PORT
• Documentation updated across multiple files to reflect the new flag and configuration options

The implementation follows a clear precedence: CLI flag > PORT environment variable > default (3000). All port parsing includes proper validation to ensure values are within valid port range (0-65535).

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The implementation is clean and follows a consistent pattern across all files. Port resolution logic includes proper validation, the precedence order (flag > env > fallback) is sensible, and all code paths correctly read from process.env.PORT. The changes are backward compatible (defaults to 3000) and documentation is comprehensive.
• No files require special attention

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