Add the agent plugin with all supporting infrastructure:

- AgentPlugin class: routes, tool collection, SSE streaming, thread mgmt
- AgentEventTranslator: converts AgentEvent to OpenAI Responses API format
- InMemoryThreadStore: per-user conversation persistence
- Config-driven agents: frontmatter markdown files in config/agents/
- Layered system prompts: auto-generated base (guidelines + app context)
  prepended to per-agent user prompt
- AgentDefinition union type: agents accept { adapter, systemPrompt }
- Auto-discovery: config files merged with code-defined agents per-name
- reloadAgents() export for future hot-reload support
- ToolProvider implementations for analytics, files, genie, lakebase
- Server plugin addExtension() for plugin-to-plugin route registration

Also add a shared defineTool() helper for ToolProvider plugins and refactor
the four built-in ToolProviders to use it:

- defineTool(): Zod-powered factory that pairs a schema with a typed handler
  and optional ToolAnnotations. Names are supplied by the enclosing registry
  key, so dotted names like `uploads.list` or `myspace.sendMessage` work
  without any special-case dispatch code.
- executeFromRegistry(): generic dispatcher that looks up an entry by name,
  validates args via safeParse, and returns an LLM-friendly error string on
  failure rather than throwing.
- toolsFromRegistry(): derives the AgentToolDefinition[] from a registry,
  generating JSON Schema from each entry's Zod schema.
- analytics, files, genie, lakebase: getAgentTools() / executeAgentTool()
  now delegate to the shared helpers. Static plugins (analytics, lakebase)
  declare the registry as a class field; dynamic plugins (files, genie)
  build per-volume / per-space entries in the constructor after runtime
  config is known.

Signed-off-by: MarioCadenas <MarioCadenas@users.noreply.github.com>