CLI: `axon run --help` should mention credentials are required

Problem

When running axon run --help, the help text doesn't clearly indicate that credentials are required to run a task. New users might try:

axon run -p "Hello world"

...and only discover at runtime that credentials are missing.

Current behavior

$ axon run --help
Create and run a new Task

Usage:
  axon run [flags]

Flags:
      --credential-type string   credential type (api-key or oauth) (default "api-key")
      --secret string            secret name with credentials (overrides oauthToken/apiKey in config)
  -p, --prompt string            task prompt (required)
  ...

The --secret flag description mentions credentials but doesn't indicate they're required.

Expected behavior

The help text should make it clear that credentials must be configured either:

1. In ~/.axon/config.yaml (via oauthToken or apiKey)
2. Via the --secret flag (referencing a pre-created Kubernetes secret)

Suggested fix

Update the description of the --secret flag and/or add a note at the bottom:

Flags:
  ...
  --secret string            secret name with credentials (overrides oauthToken/apiKey in config)
  ...

Note: Credentials are required. Configure them in ~/.axon/config.yaml or provide --secret.
Run 'axon init' to create a config file template.

Alternatively, make --secret required and add logic to auto-read from config if not provided.

Impact

• Reduces friction in the getting-started experience
• Makes credential requirements discoverable from --help alone
• Helps users understand the relationship between config file and CLI flags

Start by locating the implementation of the axon run --help output, especially the --secret flag and the axon init configuration guidance. Update the help text so credential requirements and the config or Kubernetes secret options are discoverable, then run axon run --help to verify the expected wording.