new(knative/client): kn CLI for Knative serverless (CNCF incubating)

[tannevaled]

Summary

• Adds a new pkgx pantry recipe for knative/client — the kn CLI for Knative, a CNCF incubating project for serverless workloads on Kubernetes.
• Builds the single Go binary kn from ./cmd/kn, injecting version.Version via ldflags so kn version reports the pantry-resolved version.
• Versions are tracked via GitHub releases with strip: /^knative-v/ since upstream tags use the knative-vX.Y.Z prefix; the source tarball URL is constructed accordingly.

Recipe lives at projects/github.com/knative/client/package.yml and follows the standard Go CLI pattern (compare with projects/projectdiscovery.io/nuclei, projects/cloudnative-pg.io, projects/hetzner.com/hcloud):

• build.dependencies: go.dev: '*'
• CGO_ENABLED=0, -s -w ldflags, plus -buildmode=pie on linux
• build.skip: fix-patchelf
• provides: [bin/kn]
• All four supported platforms (linux/darwin x x86-64/aarch64) — no platforms: restriction.

Test plan

• pkgx build github.com/knative/client succeeds on linux/x86-64
• pkgx build github.com/knative/client succeeds on darwin/aarch64
• kn version prints Version: vX.Y.Z matching the resolved version
• Version auto-discovery picks up new knative-vX.Y.Z releases and exposes them as X.Y.Z

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