Toggle MCP/tools per-session

### Feature summary

Allow to enable/disable tools on a per-session basis

### What problem are you trying to solve?

Save tokens by stripping tools definitions that are not required for the task

### Proposed solution

_No response_

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

No files, tests, or entry points are identified in the issue. Start by locating how sessions and MCP/tool definitions are represented, then determine how per-session enablement should be configured. Done means a session can enable or disable individual tools and unused tool definitions are stripped to reduce tokens.

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