Fix derived lifecycle for date-based release bundles

[lcawl]

Fixes #3725

AI summary

docs-builder code

• VersionLifecycleInference.InferLifecycle — ISO dates (YYYY-MM-DD) now return ga before semver prerelease parsing.
• VersionLifecycleInferenceTests.cs — unit tests for semver and date cases.
• BundleChangelogs_WithProfile_DateVersionAndLifecyclePlaceholder_WritesGaLifecycle — integration test confirming {lifecycle} + date arg → ga.

docs-builder documentation

• docs/cli/changelog/cmd-bundle.md — date row in inference table; omit / derive / hardcode patterns.
• docs/contribute/configure-changelogs-ref.md — date-based profile examples.
• docs/contribute/bundle-changelogs.md — ISO date → ga; lifecycle YAML options.
• config/changelog.example.yml — serverless profile lifecycle comment examples.

Tests

I tested by re-running the bundle command for elastic/kibana#280819:

/path/to/docs-builder/.artifacts/publish/docs-builder/release/docs-builder changelog bundle serverless-release 2026-07-22 ./docs/temp/serverless-prs.txt

Confirmed that the lifecycle now is ga instead of preview:

products:
- product: cloud-serverless
  target: 2026-07-22
  lifecycle: ga
  repo: kibana
  owner: elastic
release-date: 2026-07-25

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor Grok 4.5