cleanup(docker): remove unused ClientConfig.HTTPClient field (#684) (#693)

## Summary

Removes the \`HTTPClient\` field from \`ClientConfig\`. The field was
declared in [#681](#681) but
never read by \`createHTTPClient\` or \`NewClientWithConfig\` — a caller
setting \`cfg.HTTPClient = someClient\` got the auto-constructed
transport instead of theirs.

A repo-wide grep confirms zero field-access or struct-literal callers in
production code or tests. Only the function name \`createHTTPClient\`
and the local variable \`httpClient\` turn up in matches; the field
itself has no readers.

Closes [#684](#684).

## Design choice

The [issue](#684) presented
two options:

1. **Honor the field** — useful as a testing seam (inject fake
transport).
2. **Remove the field** — more honest about current code.

**Chose Option 2** because:

- Zero current callers in tests or production — the field is genuinely
vestigial.
- The equivalent test-seam pattern (\`SetTransportFactoryForTest\`)
already exists in \`middlewares/webhook.go\` where it's actually needed.
- Honoring the field would also require running
\`disableHTTP2AutoConfig\` on caller-supplied transports when the scheme
is non-TLS (the #668 invariant), expanding scope significantly for a
field nobody uses.
- If a future contributor needs a transport-level injection seam, they
can add it with a clear use case spelled out — better than reviving a
vestigial field.

## Test plan

- [x] \`go test ./...\` passes (full repo, 14 packages, ~56s)
- [x] \`golangci-lint run\` clean
- [x] \`go vet ./...\` clean
- [ ] CI green

Author: CybotTM

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Removed
+
+- **BREAKING (source-only, pre-1.0):** Removed unused `core/adapters/docker.ClientConfig.HTTPClient` field that was declared in [#681](https://github.com/netresearch/ofelia/pull/681) but never read — a caller setting `cfg.HTTPClient = someClient` silently got the auto-constructed transport instead of theirs. Downstream Go consumers that referenced the field in named struct literals or assignments will see a compile-time error after upgrade (semantically a no-op since the field was already ignored at runtime); permitted under SemVer for the current 0.y.z line (cf. [SemVer §4](https://semver.org/#spec-item-4)). Removing the field turns the silent footgun into a loud compile-time error rather than preserving it as a deprecated no-op. If you need a transport-level injection seam, file a feature request with the use case so the suppression of `disableHTTP2AutoConfig` on caller-supplied transports (the [#668](https://github.com/netresearch/ofelia/issues/668) invariant) can be wired in correctly. ([#693](https://github.com/netresearch/ofelia/pull/693), closes [#684](https://github.com/netresearch/ofelia/issues/684))
+
 ## [0.25.1] - 2026-05-16
 
 ### Added

core/adapters/docker/client.go

@@ -190,9 +190,6 @@ type ClientConfig struct {
 	// Version is the API version (empty for auto-negotiation)
 	Version string
 
-	// HTTPClient is a custom HTTP client (optional)
-	HTTPClient *http.Client
-
 	// HTTPHeaders are custom HTTP headers (optional)
 	HTTPHeaders map[string]string