fix(install): three quality issues affecting generated AI tooling files

[barrytarter]

Summary

Three independent generator-quality bugs in Boost v2 that surfaced via PR reviews on multiple downstream projects after composer require --dev laravel/boost:^2.0 && php artisan boost:install. All are tooling-quality issues — no behavioral or breaking changes.

#	Bug	File
1	GuidelineAssist::appPath() outputs mixed path separators on Windows (e.g., app\Http/Kernel.php)	src/Install/GuidelineAssist.php
2	@boostsnippet directive renders captions as <!-- Name --> HTML comments instead of Markdown sub-headings	src/Concerns/RendersBladeGuidelines.php
3	Skill files written without a trailing newline (SkillWriter::copyFile calls trim() then writes without restoring)	src/Install/SkillWriter.php

Bug 1 — Path separators

GuidelineAssist::appPath('Http/Kernel.php') does:

ltrim(Str::after(app_path($path), base_path()), DIRECTORY_SEPARATOR);

On Windows, app_path() and base_path() concatenate with \ while the caller-supplied sub-path uses /. The result is app\Http/Kernel.php — mixed separators that leak into generated CLAUDE.md content (notably the laravel/v12 guideline lines like app\Console/Kernel.php). Display paths in Markdown should always use forward slashes regardless of host OS.

Fix: normalize \ → / on the result.

Bug 2 — HTML comment captions

@boostsnippet("Basic Vue Page Component", "vue") ... @endboostsnippet previously rendered as:

<!-- Basic Vue Page Component -->
```vue
<script setup>...

HTML comments render as visible text in some Markdown processors, don't contribute to TOC/navigation, and are unconventional. Now renders as:

Basic Vue Page Component

<script setup>...

— proper sub-heading that integrates with the document structure.

### Bug 3 — Trailing newlines

`SkillWriter::copyFile` calls `MarkdownFormatter::format(trim(...))` and then `file_put_contents(...)`. The `trim()` strips the trailing newline; nothing restores it. Result: every generated `.md` file under `.claude/skills/`, `.cursor/skills/`, `.github/skills/`, `.junie/skills/` ends without `\n`, creating "no newline at end of file" diffs forever.

One downstream project saw this on **92 skill files** in a single Boost v2 install. Fix: append `"\n"` if missing before writing.

## Tests

- Updated existing `appPath` tests to assert forward-slash output (the previous `DIRECTORY_SEPARATOR` assertions accidentally masked the Windows bug on Linux CI).
- Added a new test covering the Windows-style mixed-separator input case.
- Updated existing `boostsnippet` tests (3 assertions) to expect `#### Name` instead of `<!-- Name -->`.
- Added a new `SkillWriter` test using a fixture that intentionally lacks a trailing newline, verifying the writer restores it.

Tests: 10 passed (23 assertions) on the touched test files

Pre-existing `ToolExecutor` test failures observed when running the full suite on Windows are unrelated and present on `upstream/main` as well.

## Test plan

- [ ] CI green on `ubuntu-latest`
- [ ] Manual: run `php artisan boost:install` on a fresh Laravel 12 project, verify generated skill files end with `\n` and `inertia-vue-development/SKILL.md` uses `#### Caption` instead of `<!-- Caption -->`
- [ ] Manual on Windows: verify `CLAUDE.md` `laravel/v12` section emits `app/Http/Kernel.php` (not `app\Http/Kernel.php`)

## Out of scope

A fourth issue surfaced — Boost generates the `volt/core` guideline saying `IMPORTANT: Activate volt-development every time...`, but the `volt-development` skill bundle in `.ai/volt/skill/volt-development/SKILL.blade.php` doesn't always materialize for projects that have `livewire/volt` installed. This needs deeper investigation into the SkillComposer detection path and is out of scope for this PR — happy to file as a separate issue.

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