Release 0.6.0

What's Changed

• feat(mif-rh): respect an existing version pin on ontology fetch by @zircote in #62
• chore(release): v0.6.0 by @zircote in #63
• chore(deps): relax sysinfo to a non-exact pin by @zircote in #64
• feat(mif-schema,mif-cli,mif-mcp): L1/L2/L3 level-floor overlay (validate --level) by @zircote in #65
• feat(mif-rh,mif-rh-cli): breaking-change detection for pinned ontologies by @zircote in #67
• feat(mif-cli,mif-mcp): standalone roundtrip/emit-jsonld/emit-markdown subcommands by @zircote in #66
• fix(mif-cli,mif-rh-cli): allow leading-hyphen values in free-text CLI args by @zircote in #71
• docs(changelog): complete the v0.6.0 entry before release by @zircote in #72

Full Changelog: v0.5.0...v0.6.0