feat(mcp): billing read tools

[andypalmi]

Summary

• Adds read-only MCP tools for team billing visibility in forge/ee/lib/mcp/tools/billing.js:
  • platform_get_team_billing: reads a team's subscription and plan details (GET /ee/billing/teams/:teamId)
  • platform_get_team_customer_portal: returns a Stripe customer-portal URL for a team (GET /ee/billing/teams/:teamId/customer-portal, which issues a 303 redirect; the tool captures the Location header and returns it as url)
• Both tools are annotated readOnlyHint: true, destructiveHint: false.
• Allow-lists team:edit in IMPLICIT_TOKEN_SCOPES['user:expert-mcp'] (forge/routes/auth/permissions.js) so expert-mcp tokens can call these owner-gated billing routes. The scope's Owner role still applies, so the tools resolve only on teams where the user is an Owner.

Closes #7683

Test plan

• eslint on changed files (also ran via commit hook's full lint)
• permissions_spec covers the implicit grant: an owner expert-mcp token may use the granted team:edit, a member is still role-gated, and a scope outside the implicit list stays denied
• Manual verification against a team with an active subscription

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.18%. Comparing base (a8f760b) to head (b9c2e9e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7725      +/-   ##
==========================================
+ Coverage   76.17%   76.18%   +0.01%     
==========================================
  Files         440      441       +1     
  Lines       23604    23615      +11     
  Branches     6285     6286       +1     
==========================================
+ Hits        17981    17992      +11     
  Misses       5623     5623              

Flag	Coverage Δ
backend	76.18% <100.00%> (+0.01%)	⬆️

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