docs: add missing v0.13.0 changelog entry

[andybons]

Summary

• Adds the missing v0.13.0 changelog entry between v0.14.0 and v0.12.0
• Covers the http-scope request body filtering feature from PR feat(proxy): add http-scope request body filtering #33, tagged 2026-06-18 by Daniel Pupius
• Documents both the body inspection feature and the keep v0.3.0 → v0.5.0 upgrade with context propagation

Test plan

• CHANGELOG.md renders correctly with v0.13.0 between v0.14.0 and v0.12.0

[claude]

Claude finished @andybons's task in 40s —— View job

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Code Review

• Gather context and read changed files
• Review CHANGELOG.md changes
• Post final review

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This is a docs-only PR adding the missing v0.13.0 changelog entry. The change is correct and well-formed.

What I checked:

• Version ordering: v0.13.0 — 2026-06-18 sits correctly between v0.14.0 — 2026-06-23 and v0.12.0 — 2026-06-12
• Section structure (### Added, ### Changed) matches the conventions used throughout the rest of the file
• PR reference [#33] is consistent with how other entries reference their source PRs
• The body inspection entry accurately describes the fail-closed semantics (non-JSON with payload → 403, bodyless → null), the RequiresBody short-circuit, and the 10 MB limit
• The keep upgrade entry correctly notes the context propagation change and the affected call sites (proxy.go, mcp.go, llmpolicy.go)

No issues found. The entry is accurate, complete, and consistent with the file's style.