docs(fdw): fix has_permission filter examples

[capybara-alt]

Fixes #3236

The documented AND has_permission = true examples fail when run: Postgres
folds = true into a bare column reference before pushdown, and the proxy
rejects has_permission in WHERE clauses.

Changes

• README.md: use has_permission IS true in the Lookup Resources / Lookup
  Subjects examples — IS TRUE is not pushed down by postgres_fdw and gets
  evaluated locally, so the query works as intended.
• Architecture.md: drop the has_permission condition from the
  handler-dispatch and cursor examples entirely — these show SQL as received
  by the proxy, where no local filtering exists and neither form is supported.

Testing

Verified against a live setup (SpiceDB + FDW proxy + Postgres 17 with
postgres_fdw): the original examples fail with operation not supported: column_ref, the updated README examples return the expected rows
(EXPLAIN VERBOSE confirms IS TRUE becomes a local Filter node), and the
updated Architecture.md queries dispatch to LookupResources/LookupSubjects.

[josephschorr]

• README.md: use has_permission IS true in the Lookup Resources / Lookup
  Subjects examples — IS TRUE is not pushed down by postgres_fdw and gets
  evaluated locally, so the query works as intended.

Is this being added solely to make sure the "missing caveat context" results are not included?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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