feat(plugin): hide tools per request with a snapshot hook

[Hona]

Adds a tool.snapshot plugin hook so a plugin can hide tools for one model request based on the session and agent. This is the missing piece for per-session availability: registrations stay Location-wide, but what a request sees can now vary.

• Fires once per model request in Tool.Service.snapshot(permissions, request), after permission filtering.
• Receives the permitted effective tool names; removing a name hides the tool from both the native list and the Code Mode catalog, so the rendered catalog and the execute runtime stay consistent.
• Hooks can only remove names. Added names are ignored, so a plugin cannot reveal a tool that permissions denied.
• Visibility only. A hidden tool still enforces its own permission if it is ever executed.
• Works through the existing PluginHooks path, so the Promise API gets it with a type addition only.

// Hide browser tools unless a desktop browser is attached to this session
await ctx.tool.hook("snapshot", (event) => {
  if (!browsers.has(event.sessionID)) event.tools = event.tools.filter((name) => !name.startsWith("browser_"))
})

flowchart LR
  A[Registered tools<br/>Location-wide] --> B[Permission filter<br/>agent ruleset]
  B --> C[tool.snapshot hook<br/>sessionID, agent, tools]
  C --> D[Native definitions]
  C --> E[Code Mode inventory<br/>catalog + execute runtime]

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interface ToolHooks {
  snapshot: {
    readonly sessionID: Session.ID
    readonly agent: Agent.ID
    /** Effective tool names advertised to this request. Remove names to hide tools; added names are ignored. */
    tools: string[]
  }
  "execute.before": ...
  "execute.after": ...
}

Why a hook rather than a per-tool predicate: it needs no change to Tool.Info, one registration covers a whole namespace, and it reuses the per-request hook infrastructure already used by execute.before. MCP catalogs keep updating live through reload(); that remains the right tool for location-wide changes.

Follow-up: the browser plugin will use this to hide its 44 Code Mode tools in sessions without an attached desktop.