fix(integrations): stop frontmatter injection gluing onto a missing trailing newline - #4570

Summary

_inject_frontmatter_flag() in ClaudeIntegration/VibeIntegration/AlquimiaAIIntegration detects the closing --- line's existing EOL and reuses it when injecting a new frontmatter key. When that --- is the file's last line with no trailing newline, the injected text is appended with no newline separator at all, producing user-invocable: true--- instead of a clean, separately-lined ---.

This corrupts the frontmatter (the closing delimiter is no longer alone on its own line), and since post_process_skill_content() chains multiple calls (user-invocable, then disable-model-invocation, then any fork-context keys), the second call's pre-scan can no longer find a second --- line at all — so every subsequent key injection is silently dropped, not just corrupted.

post_process_skill_content() runs on content from "external skill generators (presets, extensions)" per its own docstring, so a trailing newline after the closing delimiter isn't guaranteed.

>>> ClaudeIntegration._inject_frontmatter_flag("---\nname: x\n---", "user-invocable")
'---\nname: x\nuser-invocable: true---'          # corrupted delimiter
>>> # a second call on that output:
>>> ClaudeIntegration._inject_frontmatter_flag(_, "disable-model-invocation", "false")
'---\nname: x\nuser-invocable: true---'           # unchanged -- silently dropped

DroidIntegration's own copy of this exact helper already emits an unconditional \n instead of detecting/reusing the existing EOL, avoiding this bug entirely. Ported that fix to the other three implementations that share the same code.

Changes

• src/specify_cli/integrations/claude/__init__.py, vibe/__init__.py, alquimia/__init__.py: _inject_frontmatter_flag now always emits f"{key}: {value}\n", matching DroidIntegration.
• Added a regression test to each affected integration's test file, covering both the single-call corruption and the chained-calls silent-drop.

Test plan

• ruff check . clean
• New tests fail against the pre-fix code (verified via test-the-test) and pass with the fix
• Full tests/integrations/test_integration_{claude,vibe,alquimia,droid}.py suite: 185 passed, 1 pre-existing/unrelated skip