ci: revert Claude Opus 5.5 workflow update

[cquil11]

Revert #2805's six Claude Code model settings across five workflows. This restores claude-fable-5-1 in the sign-off and review workflows and claude-opus-5 in the plan and sweep workflows.

The CODEOWNER sign-off verifier failed twice after the model update with Claude execution failed: result is_error:true. Its detailed output is hidden, so the model change is a suspected cause rather than a confirmed diagnosis. After merging this revert, dispatch the verifier again for PR #3388 to check whether the prior model completes.

Validation: git diff --check passed. actionlint reports existing $/.github/workflows/... reusable-workflow references in klaud-plan.yml and run-sweep.yml; the revert does not touch those references.

[claude]

Looks good — a mechanical revert of --model string literals in five workflow files, restoring known-good model identifiers.
What was reviewed: confirmed the diff only touches six --model 'claude-opus-5-5' literals across the five listed workflows, with no changes to logic, permissions, or tool allowlists. Cross-checked klaud-plan.yml (now claude-opus-5) and klaud-candidate.yml (now claude-fable-5-1) against docs/klaud.md, which documents exactly those model names for the "PR review" and "Candidate execution" stages respectively. The other two files (claude.yml, codeowner-signoff-verify.yml) revert to claude-fable-5-1, consistent with the PR description that this reverts a prior model bump (#2805) suspected of causing sign-off verifier failures.

Extended reasoning...

The diff is limited to six --model string-literal changes across five GitHub Actions workflow YAML files, with no logic, permissions, or tool-allowlist changes. It touches no security-sensitive surface (auth, crypto, injection). Two of the six edits (klaud-plan.yml, klaud-candidate.yml) exactly match documented model assignments in docs/klaud.md; the other four are a straightforward revert to a prior model string per the PR description, addressing a suspected regression. CODEOWNERS only assigns default ownership (@ SemiAnalysisAI/core) to these paths, no specialized owner is implicated, and the bug hunt reported no findings.