Modernize CLI: deps, concurrency, and safer exec

[cin]

Summary

Builds on the Go 1.26 / client-go 0.36 dependency bump with CLI hardening and safer parallel exec behavior.

Dependencies and CI (from modernize-go-k8s-deps)

• Bump Go to 1.26 and Kubernetes libraries to v0.36.2
• Fix Dependabot (gomod + github-actions)
• Refresh PR/release workflows; run go test in CI

Correctness and UX

• Remove leftover kubeconfig debug print that polluted stdout
• Replace panics with stderr messages and os.Exit(1)
• Preserve captured stdout/stderr when an exec stream fails
• Stop treating non-empty stderr as a hard failure (align with kubectl); merge stderr into displayed output
• Exit non-zero if any pod exec fails
• Fix error reporting so per-pod failures show the real error (not <nil>)

Performance and scale

• Add -j concurrency limit (default 16; 0 = unlimited) with a worker semaphore
• Tune client-go QPS/Burst to match concurrency
• Add -timeout for per-pod exec deadlines
• Pass only pod name/namespace into workers

Exec transport and hygiene

• Prefer WebSocket with SPDY fallback via NewFallbackExecutor
• Replace manual sort.Interface with slices.SortFunc
• Drop unused ANSI color constants
• Extend unit tests; add .gitignore; clean up README (document new flags, remove stale “possible features”)

Test plan

• go test ./...
• go build
• go vet ./...
• Manual smoke against a cluster: kubectl-parallel-exec -c <ctr> -l <sel> -j 2 -- true
• Confirm failing command exits 1 and prints the real error (not <nil>)
• Confirm stderr-only successful commands are not treated as failures

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