docs(plugins): document ctx.dispatch_tool()

[r266-tech]

Summary

PR #10763 (merged today) added dispatch_tool() to PluginContext — a public, stable interface for plugin slash commands to orchestrate tool calls. The docs were not updated in that PR, so plugin authors currently have no hint that this method exists beyond the PR description.

This PR closes the docs/code drift:

1. website/docs/user-guide/features/plugins.md — adds one row to the "What plugins can do" capabilities table alongside register_command.
2. website/docs/guides/build-a-hermes-plugin.md — adds a new "Dispatch tools from slash commands" subsection right after "Register slash commands" with:
   • A realistic example (/deliver plugin calling delegate_task)
   • Full signature table
   • Runtime behavior notes for CLI mode, gateway mode, and explicit override
   • A pointer that plugins should not reach into ctx._cli_ref.agent

Content is derived from PR #10763's description and matches the actual PluginContext.dispatch_tool implementation in hermes_cli/plugins.py.

Test plan

• Markdown renders correctly (no table/tip block breakage)
• No changes to code or tests — docs-only
• Signature table matches the method's actual keyword-only parent_agent parameter