fix: make railway docs work in non-interactive mode

[m-abdelwahab]

Summary

• railway docs no longer requires a TTY — in non-interactive mode (agents, CI, piped stdout), it prints the doc URLs instead of bailing
• Drops the confirmation prompt ("Open the browser?") in interactive mode — just opens the browser directly
• Prints all three doc URLs in non-interactive mode: the docs homepage, llms.txt index, and llms-full.txt full content
• Removes the now-unused NON_INTERACTIVE_FAILURE constant from src/consts.rs

Before

$ railway docs | cat
Error: This command is only available in interactive mode

After

$ railway docs | cat
https://docs.railway.com
https://docs.railway.com/llms.txt
https://docs.railway.com/llms-full.txt

Test plan

• railway docs | cat prints all three URLs, exits 0
• railway docs in a terminal opens the browser without prompting
• railway docs --help shows updated description

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