Add --headless mode for automated installs

[nrodd]

Stacked on #54. Review that first; this PR shows only the headless diff and should be retargeted to main once #54 merges.

Summary

Adds a no-prompt mode for automation, such as CI or a hosted install that runs the wizard inside a customer's repo with credentials it already holds.

• --headless configures the codebase only:
  • installs the snippet statically when the framework is recognized
  • hands CSP, identity, analytics linkage and privacy masking to a terminal agent (Claude Code, Codex or Gemini) when one is installed
  • skips login, plugin setup, the demo and the clipboard
  • writes .subtext/install-result.json; exits 0 when the snippet is installed, 1 when it isn't, 2 with no credential
• --org <id> uses the public snippet service without a credential, so telemetry is skipped.
• --external-agent does the deterministic steps, then writes the remaining agent work to .subtext/agent-prompt.md instead of launching a local agent CLI. This is for a hosted agent (for example an Anthropic Managed Agents session) that runs the wizard itself.
• Without a TTY, Claude Code progress prints as log lines instead of a spinner.

Test plan

• npm test (103 passing, including new headless.test.ts: no-credential exit, static install with no prompts, failure without agent, --external-agent with and without a static install, --org without a credential)
• npm run typecheck
• Real run with an agent CLI installed
• Hosted run via the Managed Agents proof of concept