feat(cli): add --no-default-agents flag to kagent install

[Raghavendiran-2002]

Summary

Closes #766

Adds a --no-default-agents flag to kagent install that disables all default agents, enabling a minimal
installation without editing the helm values file or passing multiple --set flags.

kagent install --no-default-agents

- Disables all 10 default agents (k8s-agent, kgateway-agent, istio-agent, promql-agent, observability-agent,
argo-rollouts-agent, helm-agent, cilium-policy-agent, cilium-manager-agent, cilium-debug-agent)
- Uses helm --set flags so it takes precedence over any --profile value
- Can be combined with --profile (e.g. --profile demo --no-default-agents)

Files changed

- go/core/cli/internal/cli/agent/install.go — NoDefaultAgents field on InstallCfg, defaultAgents list, flag handling
- go/core/cli/cmd/kagent/main.go — register --no-default-agents cobra flag

Test plan

- All Go unit tests pass (make -C go test)
- kagent install --no-default-agents installs kagent with no agents deployed
- kagent install --profile demo --no-default-agents installs with demo profile settings but agents disabled
- kagent install (no flag) still installs default agents as before

[Copilot]

Pull request overview

This PR adds a --no-default-agents flag to the kagent install command that disables all 10 default agents via helm --set flags, enabling users to perform a minimal installation without editing values files or passing multiple individual set flags.

Changes:

• Added NoDefaultAgents boolean field to InstallCfg struct to track the flag state
• Created defaultAgents package-level slice containing all 10 agent names with supporting documentation
• Implemented logic to append helm --set flags disabling each agent when the flag is enabled, ensuring precedence over profile values
• Registered the CLI flag in the main kagent command

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
go/core/cli/internal/cli/agent/install.go	Added struct field, agent list, and flag handling logic with proper helm precedence
go/core/cli/cmd/kagent/main.go	Registered the --no-default-agents boolean flag with cobra

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