chore(migrations): remove pre-2026 migrations and rely on schema.rb

[mroderick]

What

Removes all 163 pre-2026 migrations and relies on db/schema.rb + db:prepare for fresh databases. The 13 in-window 2026 migrations remain as the only files in db/migrate/.

Why

Following the practice discussed in #2774: once migration files have been applied to all environments, db/schema.rb is the source of truth and the historical files can be removed. This keeps only the migrations in the strong_migrations window (2026 onwards).

Changes

• Deleted 163 pre-2026 migration files.
• Heroku release step changed from bundle exec rake db:migrate to bundle exec rake db:prepare so a fresh dyno loads from db/schema.rb and an existing dyno runs pending migrations.
• Updated dev setup instructions in AGENTS.md, README.md, and docs/development-setup.md to use db:prepare.
• Added a note in AGENTS.md explaining the migration cleanup.

Verification

• bundle exec rake db:create db:prepare from a scratch test database reproduces the committed db/schema.rb exactly.
• db:migrate:status on the scratch DB shows the 13 kept 2026 migrations all up.
• Full test suite: bundle exec parallel_rspec spec/ -n 3 — 1172 examples, 0 failures.

Dependencies

• Requires fix(schema.rb): remove unbacked columns added by self-check-in migrations #2776 (merged), which removed unbacked columns from db/schema.rb so the schema can be loaded cleanly.

[olleolleolle]

Grand!