feat: add agent.preset for simplified install UX

[thepagent]

Summary

Add agent.preset value that auto-configures image + command, reducing install to:

helm install agent-broker agent-broker/agent-broker \
  --set discord.botToken="$DISCORD_BOT_TOKEN" \
  --set discord.allowedChannels[0]="CHANNEL_ID" \
  --set agent.preset=codex

Presets

Preset	Image	Command
(empty/default)	agent-broker	kiro-cli acp --trust-all-tools
codex	agent-broker-codex	codex-acp
claude	agent-broker-claude	claude-agent-acp

Changes

• _helpers.tpl — 3 new helpers to resolve preset → image, command, args
• deployment.yaml — use agent-broker.image.repository helper
• configmap.yaml — use agent-broker.agent.command and agent-broker.agent.args helpers
• NOTES.txt — resolve command from preset for auth hints
• values.yaml — add agent.preset: ""

Tested

helm template --set agent.preset=codex  → image: agent-broker-codex, command: codex-acp, args: []
helm template --set agent.preset=claude → image: agent-broker-claude, command: claude-agent-acp, args: []
helm template (no preset)              → image: agent-broker, command: kiro-cli, args: [acp, --trust-all-tools]

Closes #25

[thepagent]

Wrong branch attached, recreating.