Expose `disableModelInvocation` on `session.skills_loaded` skill entries

## Summary

The `session.skills_loaded` event reports each skill's `enabled` and `userInvocable` flags, but not its **model-invocation** flag (the `disable-model-invocation` frontmatter gate). Please add `disableModelInvocation` to each skill entry so consumers can tell whether the model may auto-invoke a skill — not just whether the user can invoke it as a slash command.

## Why

`userInvocable` (user slash-command availability) and model-invocability are two independent axes:

- `userInvocable: true/false` — can the **user** invoke it as a slash command.
- `disableModelInvocation: true/false` — may the **model** auto-invoke it.

A skill can be any combination (user-only, model-only, both). Consumers today only receive `userInvocable`, so they can't distinguish a model-auto-invoked skill from a user-only one. This matters for:

- Rendering an accurate command palette / skill list (which entries are slash commands vs. silently model-driven).
- Projecting skills into surfaces that have a native model-invocation concept (for example, AHP's `SkillCustomization` already has a `disableModelInvocation` field, which currently can't be populated from this event).

The flag is already part of the skill model — it's used internally to decide whether a skill is visible to the model — so this is a surfacing change, not new behavior.

## Current shape

`session.skills_loaded` skill entries today (camelCase wire):

```jsonc
{
"name": "…",
"description": "…",
"source": "project | inherited | personal-copilot | personal-agents | plugin | custom | builtin",
"path": "…", // optional
"argumentHint": "…", // optional
"enabled": true,
"userInvocable": true
}
```

## Proposed change

Add `disableModelInvocation: boolean` to each entry:

```jsonc
{
"name": "…",
"description": "…",
"source": "…",
"path": "…",
"argumentHint": "…",
"enabled": true,
"userInvocable": true,
"disableModelInvocation": false // NEW
}
```

Surfaces to touch:

- The `session.skills_loaded` event schema (the per-skill entry shape).
- Generated types in every language target (e.g. the Rust `SkillsLoadedSkill` and the TypeScript `session.skills_loaded` data type).
- For consistency, mirror the field onto the related skill API types that also omit it today (the `Skill` / `ServerSkill` discovery shapes).
- Populate it at the producer from the skill model's existing model-invocation flag.

## Backwards compatibility

Additive boolean; existing consumers are unaffected. A sensible default for older producers is `false` (model invocation allowed), matching the default semantics of the `disable-model-invocation` frontmatter flag.

Start with the session.skills_loaded event schema and the per-skill entry types, including Rust’s SkillsLoadedSkill and the TypeScript session.skills_loaded data type. Then inspect the related Skill and ServerSkill discovery shapes and the producer’s existing model-invocation flag. Done means every listed shape exposes disableModelInvocation and the producer populates it with the existing flag, while older-producer defaults remain false.

Written by the indexing model from the issue text.