docs: Add a changelog

[harshaneel]

Summary

• Adds CHANGELOG.md. Releases previously carried no description beyond commit messages, so a tag told you nothing about what changed or what might break.
• Entries for v0.1.0 through v0.3.0 are reconstructed from git history. Each was checked against the actual diff for its tag range, not just commit subjects.
• The v0.4.0 entry covers feat: Publish a proxy-only image and support a remote upstream #12 (the :proxy image variant) and fix: Supervise both children instead of gating startup on the model #15 (the entrypoint supervision fix), and leads with the two things that can break an existing setup: an open port no longer implies a loaded model, and :proxy authenticates none of its callers.

Test Plan

An accuracy audit against git history found and fixed four problems in the draft:

• One factual error. The v0.1.0 entry claimed Anthropic support. That tag's own README says "Gemini and OpenAI", and no internal/protocol/anthropic package exists there. Anthropic arrived in v0.3.0.
• v0.3.0 under-claimed. Three of its user-visible fixes were listed, but PR feat: Add Anthropic Messages API compatibility layer #11 shipped several more across its follow-up commits. Added the server-tool handling, the count_tokens coverage, the max_tokens validation, and the duplicate/invented tool-call fixes.
• v0.4.0 was missing that the load timeout is gone entirely, not merely repaired. A reader could otherwise assume timeout-then-exit still happens.
• v0.4.0 was missing the :proxy security caveat, which is the single most consequential thing a reader of that release needs.
• Every remaining claim confirmed against code or git history, including the 41 MB size (rebuilt from fff1fdb and measured at 41.4 MB), the LK_UPSTREAM default, the credential host-pinning, the 60s to 180s start-period change and which stage it applies to, and the old entrypoint's 120s bound and jobs -p wedge, which was reproduced under dash.
• Verified the deliberate omission is sound: the redirect credential leak found during feat: Publish a proxy-only image and support a remote upstream #12's development never appeared under any existing tag, so listing it as Fixed would have falsely implied users were exposed.
• No em-dashes.

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