docs(workflow): workflows guide + troubleshooting (PR-6) + remove orphan migration

Adds the Workflows docs page, the Workflows troubleshooting section, and the
"workflows" sidebar entry in astro.config.mjs. Removes the orphan
packages/opencode/migration/20260511173437_session-metadata/{migration.sql,
snapshot.json} (a stale one-off SQL fragment unreferenced by the active
packages/core migration system and absent from net-state).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mguttmann