fix(agent): aggregate the PostgreSQL grounding column read per table (B52)

[koraysrn]

What

The composed PostgreSQL grounding capture (composePostgresCatalog()) projected one row per column. Against maxResultRows: 200, a stock image's own wide catalog refused the read before the user created anything — measured on TimescaleDB, Cloudberry and AlloyDB Omni and recorded as B52.

This change aggregates the columns per table with json_agg(json_build_object(...) ORDER BY ordinal_position) + GROUP BY table_schema, table_name, so the projection is one row per object, symmetric with the SQLite side. buildPostgresTables() now parses the aggregated columns array through a new parsePostgresColumns() that accepts both the pg-parsed array and a JSON-string transport form, and returns an empty list for malformed values rather than losing the snapshot.

Why

On those images the agent was unusable out of the box: the grounding capture was refused and every plan run answered ungrounded.

Live verification (compat profile)

Server	flat rows (before)	aggregated rows (after)
TimescaleDB latest-pg17	385	46
Cloudberry 2.1.0 (gpadmin)	479	67
Cloudberry 2.1.0 (least-privilege)	—	53
AlloyDB Omni 17.9.0 (postgres)	542	70
AlloyDB Omni 17.9.0 (least-privilege)	—	69

All under the 200-row budget, and the user's tables are named in every capture. Column order is preserved (id → customer_id → total), and the privilege model is unchanged — a least-privilege role aggregates exactly what it is allowed to see.

Notes

• The byte budget (maxResultBytes, 256 KB) remains the payload backstop; an extreme single-table column count would be refused by bytes rather than rows. Documented on composePostgresCatalog.
• A schema with more than 200 tables is still refused — B52 removes the per-column overflow, not the per-table one.
• New follow-up recorded as B76: the capture now admits the image's own extension views (AlloyDB's least-privilege role sees 67 ai.* / google_db_advisor_* views beside its 2 user tables) — not a privilege leak, but grounding noise to address separately.
• Docs: B52 removed from docs/BACKLOG.md and docs/AGENT.md; the compatibility.ts caveats for the three servers updated to match.

Verification

• bun run typecheck — pass
• bun run lint — 0 errors
• bun run knip — pass
• bun run build — pass
• Targeted suites all green: composed-sql.test.ts (79), context-snapshot.test.ts (98, incl. wide-catalog simulation + malformed/string/wide-table robustness), backlog-structure.test.ts (93), compatibility.test.ts, plan-grounding.test.ts, types.test.ts.

[codecov]

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