feat: contextual landing page

[scotwells]

Summary

Running datumctl with no arguments used to dump a wall of usage text. Now it opens a context-aware welcome screen that meets you where you are.

Logged out

If you haven't authenticated yet, the landing page explains your options with enough detail to pick the right one:

Welcome to Datum Cloud.

datumctl manages Datum Cloud resources from your terminal — DNS zones,
projects, workloads, IAM, and more.

You're not signed in yet. Pick the login style that fits your situation:

  datumctl login
    Opens a browser for OAuth sign-in, then walks you through picking
    a default context. The easiest path for everyday use.

  datumctl login --no-browser
    Prints a short code and a URL to visit on any device — no browser
    needed on this machine. Good for SSH sessions, remote servers, or
    any environment where a browser can't be launched locally.

  datumctl login --credentials ./key.json
    Authenticates as a machine account for CI/CD and automation.
    Non-interactive — no browser, no prompts.
    Requires: a machine account created in the Datum Cloud portal and
    its credentials JSON file downloaded from there. Human accounts
    cannot use this path.

Run 'datumctl --help' for the full command reference.

Logged in, no context selected

If you're authenticated but haven't picked a context yet, the landing shows your identity and nudges you to pick one:

Good morning, Jane.

  Signed in as   jane@acme.com
  Context        (none — run 'datumctl ctx use' to pick one)
  Access         2 org(s), 4 project(s)

First, pick a working context:

  datumctl ctx use       Interactive context picker
  datumctl ctx           List all available contexts
  Switch account         datumctl auth switch

Tip: 'datumctl ctx' with no arguments opens an interactive context picker.

Run 'datumctl --help' for the full command reference.

Logged in with an active context

Once a context is set, the landing greets you, confirms your identity and active project, shows access breadth, suggests task-oriented next steps, and rotates a tip on each run:

Good morning, Jane.

  Signed in as   jane@acme.com
  Context        Infrastructure (acme-corp/infra)
  Access         2 org(s), 4 project(s)

What would you like to do?

  Explore what's there   datumctl get <resource>
                         datumctl describe <resource> <name>
  Ship something         datumctl apply -f file.yaml
                         datumctl create <resource> ...
  Explore the API        datumctl api-resources
  Follow the audit trail datumctl activity
  Switch context         datumctl ctx
  Switch account         datumctl auth switch

Tip: 'datumctl apply -f -' reads YAML from stdin — great for piping from scripts.

Run 'datumctl --help' for the full command reference.

Stacked on

Stacks on #149 — depends on the session/context discovery work landed there.

Test plan

• datumctl with no active session shows the logged-out landing with all three login options and their descriptions
• datumctl when signed in but no context selected shows the identity block with (none) context and the pick-a-context suggestions
• datumctl when signed in with a context shows the greeting, identity block, task-grouped suggestions, and a rotating tip
• datumctl --help still shows full usage as before
• Tips rotate across runs (seeded by nanosecond timestamp)

🤖 Generated with Claude Code

[ecv]

Massive improvement. I also wonder about including a proper manpage.