fix(installer): remove bmad-help from Copilot Custom Agents picker (#2359)

* fix(installer): remove bmad-help from Copilot Custom Agents picker

Per @BMadCode's feedback after #2324 merged: every persona agent's
activation message already advertises bmad-help, so its picker entry is
redundant AND confusing (looks like a peer agent when it's actually the
meta-help). Removes the ALWAYS_AGENT_IDS allowlist exception that put it
there.

The toml-driven filter (the mechanism BMadCode endorsed in his PR review)
remains the sole signal: a skill is a persona iff its source
customize.toml has an [agent] section. bmad-help has no customize.toml,
so under the cleaned-up filter it's correctly excluded.

Tests: replaces the inclusion assertion in Suite 17 with an exclusion
assertion. Suite still covers persona / non-conventional persona /
workflow / meta-skill-with-`-agent-`-in-name cases.

Refs #2324

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* test: clarify Suite 17 fixture comment per PR review

The fixture creates no customize.toml at all for bmad-help, so the
exclusion path being exercised is the missing-file branch — not the
file-without-[agent]-section branch. Reword the comment accordingly.

Per @augmentcode review on #2359.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>