feat(codex): extend GPT-5.6 reasoning and fallback support

[AaronZ345]

Summary

• accept none, minimal, max, and ultra as Codex reasoning efforts
• build /reasoning and /effort usage text from the active agent instead of hardcoding four levels
• add current GPT-5.x models to the hardcoded fallback used only when catalog, cache, and API discovery return nothing
• document the eight canonical Codex reasoning levels in config.example.toml

Relation to #1547

#1547 already fixed GPT-5.x discovery from /v1/models by replacing the stale allowlist with isCodexChatModel. This PR leaves that filter unchanged.

The remaining model change here is limited to defaultCodexModels(), the final offline fallback. The duplicate GPT-5.6 API test has also been removed; the upstream codex_model_test.go suite now owns API-filter coverage.

The reasoning changes are separate from model discovery. Codex CLI 0.144 accepts the newer levels, but cc-connect previously normalized values beyond xhigh to an empty string and showed a stale four-level usage message.

Validation

• go test ./agent/codex -count=1
• go test ./core -run "TestCmdReasoning_UsageListsAgentEfforts|TestCmdReasoning_SwitchesEffortAndResetsSession" -count=1
• go test -tags no_web ./cmd/cc-connect ./core ./agent/codex -count=1
• git diff --check

[AaronZ345]

Hi @chenhg5, I’ve rebased this PR onto the latest main and all five CI checks are green. Could you please take another look and merge it when you have a chance? Thanks!