docs: remove broken markdown links to internal superpowers/ specs

The Docusaurus build for docs.agentos.sh fails with onBrokenMarkdownLinks=throw
because two files contain markdown links to the internal docs/superpowers/
tree, which is gitignored and never published. The build error chain causes
all routes including /benchmarks to 404 in production.

Fixes:
- packages/agentos/docs/memory/POSTGRES_BACKEND.md: removed the
  "See the design spec at..." line pointing at
  superpowers/specs/2026-04-26-brain-storage-abstraction-design.md
- packages/agentos/CHANGELOG.md: removed the "##### Spec" sub-section
  pointing at the same superpowers spec + plan paths (under the
  0.4.0 release notes block at line 5803-5806 of source CHANGELOG)

The plain-text "Spec: docs/superpowers/..." commit-body lines still
exist throughout the CHANGELOG (13 pairs) but are not bracket-link
markdown so they don't trigger MDX broken-link errors. Recommend
configuring semantic-release to filter superpowers/-prefixed paths
out of future generated commit-body sections to prevent this from
re-occurring on the next release.

Author: jddunn

CHANGELOG.md

@@ -5800,11 +5800,6 @@ const adapter = await createDatabase({ postgres: { connectionString } });
 const brain = await Brain.openWithAdapter(adapter, { brainId: 'agent-1' });
 ```
 
-##### Spec
-
-- Spec: [packages/agentos/docs/superpowers/specs/2026-04-26-brain-storage-abstraction-design.md](docs/superpowers/specs/2026-04-26-brain-storage-abstraction-design.md)
-- Plan: [packages/agentos/docs/superpowers/plans/2026-04-26-brain-storage-abstraction-plan.md](docs/superpowers/plans/2026-04-26-brain-storage-abstraction-plan.md)
-
 ## <small>0.2.12 (2026-04-25)</small>
 
 * Merge branch 'master' of https://github.com/framersai/agentos ([fc7cf50](https://github.com/framersai/agentos/commit/fc7cf50))

docs/memory/POSTGRES_BACKEND.md

@@ -235,5 +235,3 @@ await liveBrain.exportToSqlite('/tmp/alice-snapshot.sqlite');
 const forkBrain = await Brain.openPostgres(connStr, { brainId: 'alice-fork' });
 await forkBrain.importFromSqlite('/tmp/alice-snapshot.sqlite');
 ```
-
-See the design spec at [`packages/agentos/docs/superpowers/specs/2026-04-26-brain-storage-abstraction-design.md`](../superpowers/specs/2026-04-26-brain-storage-abstraction-design.md) for the full architecture.