Implement plugin CLI hooks

[ipspace]

Like the shell CLI hooks, the plugin CLI hooks are executed at various steps during the lab up/down process. The plugin hooks could be called before or after the CLI command is executed.

[Copilot]

Pull request overview

This PR introduces a unified CLI hook runner that adds plugin-based CLI hooks around existing shell-based CLI hooks, and wires it into the create, up, and down command flows so plugins can run pre-/post- hook logic during lab lifecycle operations.

Changes:

• Added netsim/cli/_hooks.py to execute plugin hooks before/after shell CLI hooks (pre_shell_*/post_shell_*), plus direct plugin hooks for create (output/post_output).
• Switched netlab up / netlab down to use the new hook runner instead of external_commands.run_cli_hooks.
• Documented plugin CLI hook behavior in docs/dev/plugins.md.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
netsim/cli/up.py	Replaces shell-hook execution calls with _hooks.run_cli_hooks(...) during up workflow steps.
netsim/cli/down.py	Replaces shell-hook execution calls with _hooks.run_cli_hooks(...) during down workflow steps.
netsim/cli/external_commands.py	Removes the old run_cli_hooks implementation (hook handling moved to _hooks.py).
netsim/cli/create.py	Adds plugin CLI hook execution for create (output and post_output).
netsim/cli/_hooks.py	New shared implementation for shell + plugin hook execution.
docs/dev/plugins.md	Adds documentation for plugin CLI hooks and their naming/registration conventions.

[ipspace]

@Muddyblack -- here's the pretty generic implementation of plugin hooks for CLI commands. You can hook the plugin into ~20 different places ;)

Speaking of which, it might be better to execute the VXLAN scripts in #3421 from the plugin hook?

[Muddyblack]

I also made a commit to #3421 which uses that now