docs: add changelog entry for v0.3.0

[olivrg]

Description

Roll the Unreleased changelog entries into a new [0.3.0] - 2026-06-10 section ahead of tagging the v0.3.0 release, and reset Unreleased to _Nothing yet._.

The entries cover what shipped in #56: the session-aware streamable-http upstream client (handshake validation, negotiated protocol version, SSE response parsing with guardrails, classified annotation-prime diagnostics) under Fixed, and the UpstreamForwarder → StreamableHttpForwarder compatibility alias under Deprecated.

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/ (root CHANGELOG.md)
• 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. Confirm the diff touches only CHANGELOG.md.
2. Check the [0.3.0] - 2026-06-10 section matches the scope of fix(proxy): implement session-aware streamable-http upstream client #56 (one ### Deprecated entry, four ### Fixed entries) — content is moved verbatim from Unreleased, with no edits.
3. Confirm ## [Unreleased] is reset to _Nothing yet._ and the section order follows Keep a Changelog (Deprecated before Fixed).
4. CI green (the docs/lint/format gates are the meaningful ones here).

Additional Context

Release-prep for tagging v0.3.0, following the v0.2.0 flow: this PR merges to main first, then the merged commit is tagged (git tag -s v0.3.0 -m "Helio v0.3.0") and the tag push triggers the release workflow that publishes @gethelio/proxy (npm), helio (PyPI), and the GHCR Docker image together. Minor bump per SemVer: #56 added new public API (StreamableHttpForwarder) and introduced a deprecation, both of which require at least a minor release.