feat(pg-codegen): emit per-column field decoders for projections

[devin-ai-integration]

Summary

The record decoders only accept a whole table row — every NOT NULL column present, or CoerceError. Most real queries don't return one: they project a subset, alias columns, and join. Auditing constructive-db's 280 unchecked row.x as string casts, the single biggest site (157 casts, compute/lib/module-loader/src/loaders.ts) is entirely of that shape:

SELECT am.scope, am.thread_table_name, ps.schema_name AS public_schema
FROM metaschema_modules_public.agent_module am
LEFT JOIN metaschema_public.schema ps ON ps.id = am.schema_id

so there was nothing generated it could use. Each record module now also exports <TABLE>_FIELDS, one decoder per column:

export const AGENT_RUNS_FIELDS = {
  /** `id` (uuid) */ id: (value: unknown, label = 'agent_runs.id'): string => requireUuid(value, label),
  /** `status` (run_status) */ status: (value: unknown, label = 'agent_runs.status'): RunStatus => requireRunStatus(value, label),
  /** `finished_at` (timestamptz) */ finishedAt: (value: unknown): string | null => asIsoString(value)
} as const;

A NOT NULL column's decoder is strict and takes an overridable label so a projection names its own alias (FIELDS.name(row.thread_table_name, 'agent_module.thread_table')); a nullable column's is lenient and has no label, because it cannot fail. decodeExpression was generalized to take value+label expressions instead of a key, so the whole-record decoder (record.foo, `${label}.foo`) and the field decoder (value, label) are the same definition of a column's runtime check — a test asserts they agree column-by-column over a whole row.

Also replaces the README, which still documented v1's removed class X implements X output and a generateCodeTree/pg-codegen/introspect API that no longer exists.

Link to Devin session: https://app.devin.ai/sessions/b1fb8d90ec0a4cd1bd95e3c77c6f6680
Requested by: @pyramation

[devin-ai-integration]

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