[FEATURE] Surface plugin install tips from any registered marketplace

Problem

Plugin install tips — the inline banner that suggests /plugin install <name>@<marketplace> based on file patterns or CLI tool usage — only appear to fire for plugins on claude-plugins-official. For example, the tip Claude Code already surfaces today:

Tip: Working with HTML/CSS? Install the frontend-design plugin: /plugin install frontend-design@claude-plugins-official

Plugins from any other registered marketplace, whether added via enterprise managed-settings or by explicit /plugin marketplace add, never surface as tips.

Direction

Let any registered marketplace be tip-eligible. The trust signal already exists: a marketplace is registered either by an enterprise admin (via managedSettings.allowedMarketplaces) or by the user explicitly running /plugin marketplace add. Either way, the operator has already committed to trusting it enough for /plugin install. The gate that blocks tips today is more conservative than the gate that allows installs.

Reproducer

In any environment with a non-official marketplace registered (managed-settings or user-added):

1. /plugin install <name>@<that-marketplace> succeeds.
2. Trigger any documented tip condition for that plugin (file pattern match, related CLI tool usage).
3. No tip appears.
4. Republish the same plugin under claude-plugins-official and retry — tip appears.

Why it matters

Discovery at scale. Two scenarios where this hurts:

• Enterprise. Onboarding new engineers in an org with dozens of internal plugins today means each one has to be told the exact install slug; there's no way for the org's own tooling to surface relevant ones at the right moment.
• Community ecosystem. A plugin author maintaining a public marketplace can ask users to install but can't surface contextual tips. An open-source CLI can already speak the hint protocol, but today the hint only resolves against the official marketplace — every downstream marketplace has to fork the suggestion mechanism or live without it.

In both cases the trigger mechanism already exists in the harness — only the marketplace gate is in the way. The existing per-session and per-plugin show-once limits already handle spam concerns.

Adjacent

• #36386 — docs for the existing tip mechanism