feat(pages): add internal-API adapter so pages tools work on self-host CE

Community Edition (verified v1.3.1) has no public Pages API — pages live only
in the internal web API behind Django session auth. New
plane_mcp/internal_api.py signs in with a dedicated account
(PLANE_INTERNAL_API_EMAIL/PASSWORD): CSRF token -> POST /auth/sign-in/ ->
session cookie, verified via /api/users/me/, with one automatic re-auth on
session expiry. Read-only project-pages routes (list + detail with
description_html) are wired into the compat FALLBACKS registry for
pages.list_project_pages and pages.retrieve_project_page; routes pinned from
Plane v1.3.1 sources and recorded in docs/plane-api-compat.md.

Without credentials the tools return an actionable error naming the env vars
(verified live against the local CE instance). Fallback error handling now
surfaces any adapter failure cleanly. 85 unit tests pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>