chore: remove seitask runner + Workflow scenarios (cutover)

[bdchatham]

What

The Go-native integration harness has replaced the seitask-runner + Chaos-Mesh Workflow nightly (harness CronJobs deployed in platform#1207). This deletes the legacy machinery — 7k lines removed:

• cmd/seitask/, internal/seitask/, internal/runner/, internal/taskruntime/, runner/, scenarios/
• the seitask image-build step in ecr.yml; the runner* Makefile targets + RUNNER_IMG
• the committed xreview ledgers (process artifacts)

Survives: internal/keygen (general key derivation, used by TestRelease); the SeiNodeTask CRD + controller (now driven by the SDK's RunTask).

Safety

• Verified no surviving package imports internal/seitask/internal/runner/internal/taskruntime (only comment mentions remain).
• go build ./..., go vet ./..., go test ./..., and go test -c -tags integration ./test/integration/ all pass.
• Companion platform PR removes the legacy nightly CronJobs/dirs + the sei/seitask-runner ECR repo + IAM.

🤖 Generated with Claude Code

[cursor]

PR Summary

Medium Risk
Large deletion of the old nightly path; safe if platform cutover and CronJobs already use integration-harness, but any remaining references to seitask-runner or Workflow YAML would break until cleaned up (e.g. .dockerignore still mentions runner/scenarios).

Overview
Removes the legacy seitask monolith (cmd/seitask, internal/seitask, internal/runner, internal/taskruntime), runner templates/RBAC, and Chaos Mesh Workflow scenario assets, now superseded by Go integration tests (test/integration, integration-harness image).

CI/build: Drops the sei/seitask-runner ECR build from .github/workflows/ecr.yml; Makefile loses runner / runner-image / runner-push and RUNNER_IMG.

RBAC/docs/CRD copy: Strips chaos-mesh.org workflows / workflownodes from manager roles; updates SeiNodeTask docs to describe fan-out via the orchestrating caller instead of seitask-runner; internal/keygen comments point at the integration harness for Secret wiring.

Kept: SeiNodeTask CRD + controller, internal/keygen, and nightly coverage via compiled integration tests and sdk/sei RunTask.

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