docs: changelog for v0.22.0

[andybons]

Release entry for #64, which shipped without one. The v0.22.0 tag will move to this commit so the tagged tree carries its own entry, matching every prior release.

[claude]

Claude finished @andybons's task in 33s —— View job

---

Reviewing PR #65

• Gather context (PR is merged, 1 file changed: CHANGELOG.md +6/-0)
• Review the changelog entry
• Post final review

Summary

This is a docs-only PR adding the missing v0.22.0 release entry to CHANGELOG.md, backfilling the entry for #64 which shipped without one.

I checked the entry's technical claims against the current code and they hold up:

• getCredentialsForRequest (proxy/proxy.go:1548) and its withRequestedStatics helper (proxy/proxy.go:1609) match the described merge behavior (static credentials on other headers are merged into a resolver's result only when the client asked for them by header).
• proxy/resolver_static_merge_test.go exists and covers this behavior.
• Formatting/style is consistent with prior release entries (heading level, date format, bold summary line, file references).

No functional code changes, no security-sensitive surface touched, nothing to flag.