CLI: add runtime command namespace

[tnez]

Parent: #103

Depends on: runtime boundary design (#105)

Context

The public entry point for runtime packages should be the existing Agentic CLI. Users should not have to learn a separate command first.

The first visible step is adding a runtime command namespace, even before the local runtime package is fully implemented.

Goal

Add the agentic runtime ... command namespace to core with clear help text and minimal safe behavior.

Candidate commands

Initial surface, subject to the runtime boundary design:

agentic runtime list
agentic runtime add <name>
agentic runtime init <name>
agentic runtime run [target]
agentic runtime status

The first PR may implement only a subset if the boundary design recommends that.

Behavior expectations

• Commands should explain that runtimes are optional packages.
• Unknown runtime names should produce actionable errors.
• If local is not installed yet, the CLI should tell the user what package/command is needed.
• The command parser should follow existing Agentic CLI conventions.
• JSON/human output behavior should follow existing CLI patterns.

Non-goals

• Do not implement package-manager install/discovery here unless the boundary design makes that the smallest coherent slice.
• Do not create the local runtime package here.
• Do not implement Pi integration here.
• Do not add Cloudflare behavior here.

Acceptance

• agentic runtime --help or equivalent command help exists.
• Runtime subcommands are routed through the existing CLI dispatch table.
• Tests cover command parsing/error behavior.
• The command namespace is documented enough for the next runtime package issue to build on it.

[tnez]

Handoff after #104

#104 has merged via #113. The repo is now a monorepo with the published core package at packages/agentic and a private workspace root.

Start #106 from remote main, not local main:

git fetch origin --prune
git switch -c feat/106-runtime-cli-namespace origin/main

Useful files after the move:

• Core CLI entrypoint: packages/agentic/src/cli/main.ts
• Command modules: packages/agentic/src/cli/commands/*.ts
• CLI tests: packages/agentic/src/cli/main.test.ts and command tests under packages/agentic/src/cli/commands/
• Shared types: packages/agentic/src/types.ts
• Root scripts delegate into packages/agentic

Scope for #106 should stay narrow:

• Add the runtime command namespace and help/error behavior.
• Route subcommands through the existing CLI dispatch style.
• Add tests for command parsing/help/unsupported behavior.
• Do not implement package install/discovery yet; that belongs to CLI: support runtime package install, discovery, and delegation #107.
• Do not create @tnezdev/agentic-runtime-local; that belongs to Runtime local: create @tnezdev/agentic-runtime-local package #108.
• Use Design: runtime package boundary #105's boundary comment as the design source of truth.