feat(mcp): opt-in group-based tool gating

[KoscheiiB]

feat(mcp): opt-in group-based tool gating

Summary

Adds an optional way to hide MCP tools by group or by name through
environment variables, so a client can expose only the subset it needs
and save context. Gating is opt-in: with no configuration every tool
stays visible and current behavior is unchanged.

Motivation

The README already carries a context-window warning: "This MCP provides
39 tools. Disable it when not using NotebookLM to preserve context."
Today the only lever is all-or-nothing (toggle the whole server off with
@notebooklm-mcp). A query-first client that wants the read and chat
tools but not the studio, research, or management tools has no way to
trim the set without editing code.

This change gives that lever without changing the default for anyone.

What changed

• src/notebooklm_tools/mcp/tool_groups.py (new): a read/manage taxonomy
  mapping every registered tool into exactly one named group, plus the
  resolution logic and an apply(mcp) entry point.
• src/notebooklm_tools/mcp/server.py: calls tool_groups.apply(mcp)
  once at the end of _register_tools(), after all tools are registered.
• tests/test_tool_groups.py (new): unit tests for env resolution,
  precedence, the no-op path, and taxonomy disjointness.
• README.md, docs/MCP_GUIDE.md: document the three environment
  variables and add a "Selective tool exposure" section with examples
  and the full group list.

Behavior

Three environment variables, resolved in this order (later wins for a
given tool):

1. NOTEBOOKLM_DISABLED_GROUPS: comma-separated group names to hide.
2. NOTEBOOKLM_DISABLED_TOOLS: comma-separated single tool names to hide.
3. NOTEBOOKLM_ENABLED_TOOLS: comma-separated tool names to re-enable,
   overriding the two above.

Tools are hidden through FastMCP's visibility transform
(mcp.local_provider.disable(names=...)), so nothing is unregistered,
only hidden. Unknown group names are ignored. With none of the variables
set, apply() is a no-op and all 39 tools remain visible.

Usage

# Query-first setup: hide mutating groups, keep read + chat tools
export NOTEBOOKLM_DISABLED_GROUPS="notebooks_manage,sources_manage,studio,research,sharing,notes"

# Hide one extra tool, but keep studio_status from an otherwise-hidden group
export NOTEBOOKLM_DISABLED_TOOLS="tag"
export NOTEBOOKLM_ENABLED_TOOLS="studio_status"

Testing

• uv run pytest tests/test_tool_groups.py: 9 new tests pass.
• Full suite: no regressions from this change (one unrelated failure in
  tests/services/test_sources.py exists on main before this branch;
  reported separately, not touched here).
• uv run ruff check . and uv run ruff format --check .: clean.
• Manual MCP verification: started the server and listed tools through
  the client. With no env vars set, 39 tools are visible. With
  NOTEBOOKLM_DISABLED_GROUPS set to the six manage/studio/research/etc.
  groups, the client-facing list drops to the read and chat core with no
  disabled tool leaking through.

This feature does not call the NotebookLM API; it only changes which
already-registered tools are advertised to the MCP client. There is no
CLI surface for it.

Notes

• No version bump (leaving versioning to the maintainer).
• No new dependencies (standard library only).
• Additive and opt-in: existing deployments see no change unless they set
  one of the new variables.