Update README: setup, multi-line input modes, structured output

[shauryajain21]

Summary

Bring the README in line with the 0.6.0 CLI:

• Document linkup setup (was missing from commands table)
• Document --clipboard, --file, stdin, and interactive paste modes
• Document structured output type with --schema-file / --schema examples
• Add aliases column (s, f, c) to commands table
• Note LINKUP_API_KEY env var precedence and Linux clipboard prerequisites

Fixes

• --deep → --depth deep (the old flag was broken)
• "Get sources" description corrected — --output searchResults returns raw results, not AI sources
• Lead install instructions with pipx since plain pip hits "externally-managed-environment" on modern macOS/Linux

Test plan

• All commands and aliases referenced in the README resolve in the installed CLI
• All flags documented match linkup search --help output