Enable the refrakt CLI to discover and load commands from installed packages. Currently all commands are hardcoded in packages/cli/src/bin.ts. This adds a plugin system so @refrakt-md/plan (and future packages) can register subcommands under a namespace (refrakt plan <cmd>).

When a plugin package is not installed, the CLI prints install instructions and exits cleanly. The same pattern could be reused by @refrakt-md/docs (refrakt docs) or @refrakt-md/storytelling (refrakt story) in the future.

Acceptance Criteria

CLI discovers installed packages that export a cli-plugin entry point
Plugin commands are registered under their declared namespace (refrakt plan <cmd>)
When a plugin package is not installed, CLI prints install instructions and exits
@refrakt-md/plan exports cli-plugin.ts with command registrations for all 8 commands
Existing commands (inspect, contracts, validate, etc.) are unaffected
Tests for plugin discovery, registration, and missing-package fallback

Approach

Add a plugin discovery step to packages/cli/src/bin.ts that runs after built-in command matching fails. Check for a cli-plugin export from known package prefixes (@refrakt-md/*). Each plugin exports a { namespace, commands } object. Route refrakt <namespace> <subcommand> to the matching handler.

The @refrakt-md/plan package adds a cli-plugin.ts entry point alongside its existing rune exports.

References

SPEC-022 (Plan CLI)
packages/cli/src/bin.ts (current CLI entry point)