docs: add changelog entry for v0.4.0

[olivrg]

Description

Cuts the changelog for the v0.4.0 release: moves the Unreleased entries (tool definition drift detection, shipped in #59 for #25) into a ## [0.4.0] - 2026-06-11 section, per the maintainer notes at the top of CHANGELOG.md. Same pattern as the v0.3.0 cut (#57).

This PR is docs-only and closes no issues. Once it merges, the v0.4.0 tag will be pushed on the merge commit, which triggers the release workflow (npm, PyPI, Docker/GHCR + cosign, SBOM, GitHub Release).

References: #25, #59, #60 (deferred follow-up shipping with this release's known-limitation note).

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (no functional changes)
• Documentation
• CI / build / tooling

Packages Affected

• packages/proxy
• packages/dashboard
• packages/python-sdk
• Root config / monorepo tooling
• docs/
• examples/

Checklist

• I have read CONTRIBUTING.md
• My code follows the existing style (ESLint + Prettier pass)
• TypeScript strict mode — no any types or @ts-ignore without justification
• I have added or updated tests for my changes
• All CI checks pass (pnpm secrets:scan, pnpm docs:check:ci, pnpm audit --audit-level=high, pnpm build, pnpm lint, pnpm format:check, pnpm typecheck, pnpm test)
• I have updated documentation if this changes user-facing behavior
• Commit messages follow Conventional Commits (e.g. feat:, fix:, docs:)

How to Test

1. Review the CHANGELOG diff: the only change is the ## [Unreleased] heading becoming ## [0.4.0] - 2026-06-11 — no entries added, removed, or reworded.
2. Cross-check the section's claims against what actually shipped in feat(proxy): tool definition drift detection (on_tool_drift) (#25) #59 (policies.on_tool_drift, block default, baseline-annotation evaluation, aggregate exclusions).
3. Confirm no Unreleased content remains above the new section, matching the post-cut state of previous releases (v0.3.0 and earlier).

Additional Context

This is the release-cut PR for v0.4.0 — a minor bump because the release adds a new config surface and a conservative behavior change (on_tool_drift defaults to block; drifted tools are denied until restart or upstream revert).
The version number itself lives nowhere in the repo; it is injected from the git tag by scripts/set-release-version.sh during the release workflow.