build(db): clean dist before drizzle generate (#11241)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Database migrations are generated by drizzle-kit, which reads the
schema from the db package's built `dist/schema/*.js`
> - `tsc` never deletes stale outputs, so a long-lived checkout keeps
compiled schema files whose sources were deleted long ago
> - A `generate` run in such a checkout sees those ghost tables and
sweeps phantom `CREATE TABLE` statements into an unrelated migration
> - This pull request makes `generate` clean `dist` before building, so
drizzle always diffs against exactly the current schema sources
> - The benefit is that no contributor can accidentally resurrect
deleted tables inside a new migration

## Linked Issues or Issue Description

**What happened?**

Running `pnpm --filter @paperclipai/db generate` in a months-old
checkout produced a migration re-creating `cloud_upstream_connections`,
`cloud_upstream_runs`, and `company_secret_pools` — tables whose schema
sources were deleted in #10507. The compiled copies were still in
`dist/schema/`, and `drizzle.config.ts` reads the schema from `dist`, so
drizzle treated them as new tables missing from the snapshot.

**Expected behavior**

`generate` diffs the current schema sources only; deleted tables can
never reappear in a generated migration.

**Steps to reproduce**

1. Build the db package, then delete a schema source file without
cleaning `dist`.
2. Run `pnpm --filter @paperclipai/db generate`.
3. The generated migration re-creates the deleted table.

## What Changed

- `packages/db/package.json`: the `generate` script runs `pnpm run
clean` before `tsc`, so the drizzle-kit input is always a fresh build of
the current sources.

## Verification

- In a checkout carrying stale `dist/schema/cloud_upstreams.js` /
`company_secret_pools.js` artifacts, `generate` produced a phantom
migration before this change; after a clean build it reports "No schema
changes, nothing to migrate". With this change the clean happens inside
`generate` itself.

## Risks

- Low risk: `generate` is a developer-only script; the change only adds
the existing `clean` step ahead of the existing build, at the cost of a
full rebuild per generate.

## Model Used

- Claude Fable 5 (`claude-fable-5`) via Claude Code CLI, with extended
thinking and tool use.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

packages/db/package.json

@@ -42,7 +42,7 @@
     "build": "pnpm run check:migrations && tsc && cp -r src/migrations dist/migrations",
     "clean": "rm -rf dist",
     "typecheck": "pnpm run check:migrations && tsc --noEmit",
-    "generate": "pnpm run check:migrations && tsc -p tsconfig.json && drizzle-kit generate",
+    "generate": "pnpm run check:migrations && pnpm run clean && tsc -p tsconfig.json && drizzle-kit generate",
     "migrate": "pnpm run check:migrations && tsx src/migrate.ts",
     "seed": "tsx src/seed.ts"
   },