feat(cli): add --json to session kill/restore/rename/cleanup

[Ayush7614]

Summary

• Align mutating session commands with ls/get/claim-pr machine-readable output
• Scripts can drive the full session lifecycle without scraping prose

Test plan

• go test ./internal/cli/ -run 'TestSessionKill_JSON|TestSessionRestore_JSON|TestSessionRename_JSON|TestSessionCleanup_JSON'
• Spot-check prose paths still work without --json

[somewherelostt]

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• @illegalcall
• @Pulkit7070

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