chore: prepare 0.3.0 release

[jlitola]

Summary

• Bump package/plugin metadata to 0.3.0 for the minor release.
• Refresh release-facing docs/help for OpenCode, indexed tools, changelog no-data behavior, and search defaults.
• Set unified search default limit to 10 for both CLI and MCP to reduce token usage.

Verification

• bun test
• bun run typecheck
• bun run lint
• bun run build
• npm pack --dry-run
• Production CLI smoke: example, languages, search, pkg info/vulns/deps/changelog, docs list/read, code files/read/grep, feedback
• Production MCP smoke: get_example, search_language, feedback, search, search_status, docs_list, docs_read, pkg_info, pkg_vulns, pkg_deps, pkg_changelog, code_files, code_read, code_grep