Fix PostgreSQL constraint migrations and refresh LQL docs

[MelbourneDeveloper]

TLDR

Fix PostgreSQL migration constraint handling, add LQL IN (...) predicate support, and refresh the LQL documentation site/deploy shape.

What Was Added?

• Added repo-local .agents/skills/* skill definitions and converted .claude/skills/* files to pointers.
• Added SchemaConstraintNames plus PostgreSQL migration tests for named column check constraints and constraint-backed unique index drops.
• Added vendored React/ReactDOM browser assets for the reporting React renderer.

What Was Changed or Deleted?

• LQL now emits SQL for lambda in (...) membership predicates, used by function body LQL/RLS predicate flows.
• PostgreSQL migration diffing now detects unique constraints and column/table check constraints, preserves inspected constraint names, and drops constraint-backed indexes by dropping the owning constraint when needed.
• The LQL website deploy workflow now builds the Eleventy docs site, publishes the Blazor transpiler under /playground/, and uploads the Eleventy _site artifact.
• The LQL Eleventy site was consolidated into a single docs-focused landing page; old per-topic generated/source docs pages and the standalone playground source were removed.
• Updated LQL and migration specs to document in (...), named column check constraints, unique constraint inspection, and constraint-backed index drops.

Related issues: #48, #49, #50, #51.

How Do The Automated Tests Prove It Works?

• make ci completed with exit code 0 locally during PR preparation.
• PostgresColumnCheckConstraintTests.ApplyYaml_WhenColumnCheckConstraintHasName_CreatesNamedPostgresConstraints proves named YAML column checks materialize as PostgreSQL constraints and are idempotent after inspection.
• PostgresDropConstraintBackedIndexTests.DropIndex_WhenIndexBacksUniqueConstraint_DropsConstraint proves destructive unique removal drops the owning PostgreSQL constraint instead of failing on DROP INDEX.
• PostgresDropConstraintBackedIndexTests.Calculate_WhenUniqueConstraintSchemaConverged_DoesNotDropBackingIndex proves inspected unique constraints do not cause false destructive drops.
• TranslatePostgresBody_ExistsPipelineWithInList_EmitsInPredicate proves LQL body predicates emit IN (...) SQL.
• InvalidCharacter_ShouldReturnLexerError proves lexer errors surface as structured LQL errors with positions.

Spec / Doc Changes

• Updated docs/specs/lql-spec.md for in (...) predicate support.
• Updated docs/specs/migration-spec.md for named column check constraints, PostgreSQL unique constraint inspection, and constraint-backed index drops.
• Reworked LQL website docs/navigation/deploy output and refreshed the Blazor playground mount path.

Breaking Changes

• None