v0.6.0: structured output + polish for official release

[shauryajain21]

Summary

• Add --output structured with --schema-file / --schema flags for JSON-schema-constrained results
• Bump to 0.6.0; version now sourced from package metadata (single source of truth)
• Repo URLs in pyproject.toml updated to LinkupPlatform
• Better clipboard error messages (specific tool-missing hints; Wayland wl-paste fallback)
• Misc cleanup: dead default-fallback code in search, misleading render_js comment in fetch

Test plan

• linkup --version → 0.6.0
• linkup search "..." -o structured --schema-file schema.json → valid JSON matching schema (tested with Inception movie schema)
• linkup search "..." -o structured --schema '{...}' → inline schema works (tested with iPhone 16 Pro price/date)
• linkup search "..." (sourcedAnswer) still works
• linkup search "..." -o searchResults still works
• Error paths: missing schema, invalid JSON schema, missing file, schema with wrong output type