Finalize the 0.2.0 release commit

[ChoMinGi]

What changed

Finalize the repository state for 0.2.0 without adding feature code:

• set the reactor, both module parents, both examples, and all three consumer projects to 0.2.0;
• date the changelog and keep an empty Unreleased section;
• switch README and security policy text from candidate wording to durable public-release wording;
• fix the reproducible build timestamp at 2026-07-26T00:00:00Z.

The JDK-proxy @ObserveGeneration edge case is tracked separately in #7 for 0.2.1 and is intentionally not mixed into this release commit.

The exact-SHA Langfuse export and API read-back must run after this commit is merged to main. That gate needs the final merge SHA and credentials, so it is not claimed in this PR.

Testing

• ./mvnw -B -ntp clean verify
• quality profile with dependency convergence, licenses, SBOM, and SpotBugs
• Spring AI consumers with 1.0.9 and 1.1.8
• LangChain4j consumers with 1.0.0 and 1.18.0
• both examples and the core prompt consumer against locally installed 0.2.0 artifacts
• Maven evaluation confirmed all eight release and consumer coordinates resolve to 0.2.0
• git diff --check and independent release/runbook audits

Checklist

• ./mvnw -B -ntp clean verify passes
• Applicable independent builds under consumer-tests pass after a local reactor install
• Tests cover the changed behavior
• Public API or user-facing behavior is documented
• No credentials or captured model data are included