Allow customizing the startup greeting text

Feature request

Allow users to customize or replace the startup greeting/banner text that Claude Code displays when a session begins.

Use case

When running Claude Code with custom agent mandates (via --agent or .claude/agents/), the agent's first-turn behavior is fully configurable through CLAUDE.md and SessionStart hooks — but the CLI's own greeting text that appears before the agent speaks is hardcoded. For users building multi-agent systems or domain-specific workflows, the generic greeting creates a disconnect between the tool's identity and the agent's identity.

Proposed solution

A configuration option (in .claude/settings.json, a CLI flag, or a per-agent setting) that lets users specify custom greeting text, or suppress the default greeting entirely so the agent's first response serves as the greeting.

Something like:

{
  "greeting": "Welcome to HAAK." 
}

or per-agent in the agent markdown frontmatter:

greeting: "Welcome to HAAK."

or greeting: false to suppress it.

Related issues

• #2254 — Disable the welcome banner (suppression only)
• #50918 — Hide loaded context files banner on startup

This request is specifically about replacement/customization, not just suppression.