feat(mcp): notification and self-service read tools

[andypalmi]

Summary

Adds Phase 1 read-only MCP tools for user self-service (Story 5, issue 5.9-a).

Tool file: forge/ee/lib/mcp/tools/user.js

Tools added:

• platform_get_current_user - GET /api/v1/user
• platform_list_notifications - GET /api/v1/user/notifications (paginated)
• platform_list_own_invitations - GET /api/v1/user/invitations

All three are tagged readOnlyHint: true, destructiveHint: false.

Scopes allow-listed in IMPLICIT_TOKEN_SCOPES['user:expert-mcp'] (forge/routes/auth/permissions.js):

• user:read
• user:edit

Write, delete, and admin-only endpoints from the parent issue (5.9-b/c/d) are out of scope for this PR, as are the excluded sensitive self-service routes (change_password, account deletion, MFA, tokens).

Closes #7702

Test plan

• eslint clean on changed files
• CI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.42%. Comparing base (103f9ae) to head (0438f64).

Additional details and impacted files

@@                        Coverage Diff                        @@
##           feat/mcp-tools-shared-schemas    #7718      +/-   ##
=================================================================
+ Coverage                          75.41%   75.42%   +0.01%     
=================================================================
  Files                                426      427       +1     
  Lines                              22551    22563      +12     
  Branches                            5949     5949              
=================================================================
+ Hits                               17006    17019      +13     
+ Misses                              5545     5544       -1     

Flag	Coverage Δ
backend	75.42% <100.00%> (+0.01%)	⬆️

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