feat: route jenkins/circleci through cigen, retire template generators (#804)

[intel352]

Summary

PR 2 of 3 for #804. Routes the plugin's jenkins and circleci platforms through the workflow engine's config-derived cigen renderers (shipped in workflow v0.68.0, PR #810) and retires the legacy text/template generators. All four step.ci_generate platforms are now config-derived from the same CIPlan.

Satisfies acceptance #2: step.ci_generate for jenkins/circleci routes through cigen; jenkins/circleci template generators removed.

Changes

• go.mod — bump github.com/GoCodeAlone/workflow v0.67.0 → v0.68.0 (first release with cigen.RenderJenkins/RenderCircleCI). plugin.json minEngineVersion → 0.68.0.
• internal/generator.go — the platform switch routes all four platforms through cigen (4-way render switch). Deleted: the registry map, the Generator interface, the legacy default: template branch, and the internal/platforms import.
• Deleted internal/platforms/ (9 files) — jenkins/circleci template generators plus the GHA/GitLab template files that were already off the production path since feat: smart GHA/GitLab generation via cigen (PR5/7) #18 (their tests were the only remaining referees). Intentional full-package retirement.
• internal/generator_test.go — replaced the two _TemplateUnchanged tests with _CigenMarkers tests (real ExecuteCIGenerate → config-derived markers: secret wiring, wfctl migrations up, smoke, wfctl infra apply; absence of go test/wfctl deploy --image/docker build per ADR 0044). Path-safety preserved via a direct validateRelativeOutputPath unit test (the registry-injection seam is gone); deleted staticGenerator/registerTestGenerator.
• integration_test.go — TestIntegration_ExecuteCIGenerate_JenkinsCircleCI drives the real ExecuteCIGenerate for both platforms across the external package boundary (acceptance fix: use released workflow dependency #2 behavior proof). Existing pipeline-wiring mock tests retained.
• plugin.json — version 0.1.6 → 0.2.0 (behavior change: jenkins/circleci now config-derived).
• README.md — corrected (all four platforms config-derived).

Verification

• go test ./... — PASS (root + internal). _CigenMarkers for all 4 platforms + the real-ExecuteCIGenerate integration test green.
• go vet ./... — clean. golangci-lint run --new-from-rev=origin/main ./... — 0 issues.
• Plugin builds in the wfctl-discoverable layout (<dir>/ci-generator/ci-generator + plugin.json).

Sequencing

Depends on workflow v0.68.0 (merged + tagged). After merge: tag plugin v0.2.0.

Note

Copilot review intentionally not requested (broken). Merge gate = adversarial review clean + two-stage code review APPROVED + CI green.

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