chore(release): prepare v0.2.0

[owainlewis]

Summary

• bump Cortex and Cargo.lock to 0.2.0
• add grounded v0.2.0 release notes
• update stable-version examples
• require versioned release notes and publish them as the GitHub Release body

Why

The v0.2 editor surface is complete and the final repository, code, performance, terminal, security, packaging, and Nightly audits are clean. This prepares the next stable release without including unfinished v0.3 roadmap items.

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test --all-targets (317 unit tests and 5 signal-cleanup tests)
• cargo test --all-targets -- --ignored (4 performance tests)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets
• cargo build --release --target aarch64-apple-darwin
• actionlint 1.7.12 on the release workflow
• independent review with all findings resolved
• binary reports cortex 0.2.0
• two package runs produced byte-identical thin-arm64 archives

Release plan

After this PR merges with protected CI green, tag the exact merge commit as v0.2.0. Wait for the tag-triggered Release workflow, then verify the archive, checksum, signed provenance, architecture, release notes, installer resolution, and binary version.