fix(migrations): harden v0.29.1 on PGLite + guard effective_date index creation

[DrBaher]

Summary

This fixes two migration hardening issues seen on PGLite upgrades:

1. v0.29.1 orchestrator used an engine before connecting

   • Could fail with: PGLite not connected. Call connect() first.
   • phaseBBackfill and phaseCVerify now call connectWithRetry(...) before DB work and disconnect() afterwards.

2. Schema bootstrap could reference effective_date too early

   • On partially-upgraded brains, creating pages_coalesce_date_idx could fail if pages.effective_date was not yet present.
   • Index creation is now guarded by a column-existence check in schema SQL.

Files changed

• src/commands/migrations/v0_29_1.ts
• src/core/schema-embedded.ts
• src/core/pglite-schema.ts
• src/schema.sql

Verification

• bun run typecheck
• Local reproduction previously stuck at partial 0.29.1; with this change, migration completed and doctor advanced from unhealthy to warnings with schema at latest.

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[garrytan]

Thanks for the contribution! This is being closed because it was superseded by #740 in #776 (PGLite hardening covered by UNSAFE_TRANSACTION + connect fix). Real fix shipped; your name will appear in the v0.30.3 release notes attribution where applicable. Thank you again.