[FEATURE] Allow using custom MCP tools in Plan mode

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

In Plan mode, when my prompt implies use of a custom MCP tool configured on my machine (e.g. JIRA MCP tool), the Plan tool does not attempt to use the MCP tool for fetching external information. Instead, it just looks through project files in an attempt to get some idea. This can often lead to suboptimal plans, because the ticket in task tracker may contain crucial information. E.g. if I say "Please work on the ticket FOO-123", the Plan tool will not be able to understand the request at all without access to an MCP tool.

Proposed Solution

It would be handy if Plan was e.g. able to access an external task tracker like JIRA via user-configured MCP tool. If Plan tool was able to leverage the custom MCP tools, it would be possible to prompt Claude with "Please work on the ticket FOO-123" in plan mode and expect it to do the rest.

Alternatively, before calling the Plan tool, the agent could enter an additional preparation phase where it would fetch necessary information from external resources - after this, the agent can finally trigger the Plan tool with a sufficient prompt to do the rest.

Alternative Solutions

Currently I try to simulate a two-phase approach by myself:

1. In normal mode: "Please read the ticket FOO-123 and gather necessary context. Do not make any changes - I will enter the Plan mode after this and ask you to come up with a solution."
2. After that, in Plan mode: "Please start working on FOO-123 based on your knowledge of the ticket".

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

High-level scenario:

1. In Plan mode: prompt "Please start working on JIRA ticket FOO-123." with no extra context.
2. Result: agent gathers information on the JIRA ticket based only on its number, and then comes up with a proper implementation plan.

Additional Context

_No response_