remove: drop all CircleCI support

[zuchka]

Summary

Removes CircleCI from DING entirely. No more recipe, no more runctx auto-detection, no more docs mentions.

Files changed: 8 files, +4 / -132 lines.

• `internal/runctx/runctx.go` — drop the `CIRCLECI=true` detection case
• `internal/runctx/runctx_test.go` — drop `TestNew_DetectsCircleCI`; remove `CIRCLE_*` from `clearCIEnv`
• `docs/recipes/circleci.md` — deleted
• `docs/recipes/index.md` — drop CircleCI row
• `mkdocs.yml` — drop CircleCI nav entry
• `README.md` — drop from runner table + CI/CD recipes list
• `docs/configuration.md` — drop from "Platform-specific examples" pointer
• `docs/recipes/jenkins.md` — drop "/ CircleCI" reference in a Tradeoffs bullet

Breaking change

Existing users with rules that match `runner=circleci` or rely on CircleCI-derived run-context labels (`repo`, `branch`, `commit`, `job` from CIRCLE_* env) will see those alerts now report `runner=local` instead. This is a feature regression by design.

Background

An attempted Tier-2 CircleCI orb (`ding-labs/ding-circleci-orb`) hit unrecoverable CCI auth/onboarding friction for new GitHub orgs — webhook validation errors, CLI-vs-UI integration mismatches, namespace claim path broken. Rather than ship a half-supported integration, removing CircleCI entirely until the DevOps story improves.

Test plan

• `go test -race ./...` — full suite green after removal
• `grep -rn -i "circleci\|CIRCLE" --include=".go" --include=".md" --include=".yml" --include=".yaml" --include="*.example"` — zero remaining mentions in non-superpowers, non-auto-generated files

Post-merge

• Tag `v0.12.0` (breaking change per established convention).
• Memory `project_ding_wedge_status.md` — note CircleCI removed; remove the orb-resume-condition framing.
• External repo `ding-labs/ding-circleci-orb` — pending decision (delete entirely, or leave dormant).

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