mig: add is_default flag to assistants

[simplesagar]

Summary

Stacked on #3123. Migration only — first PR in the AGE-2631 stack (replace AI Insights with a per-(default-)project assistant).

Adds an is_default boolean to the assistants table plus a partial unique index so each project has at most one default (non-deleted) assistant. This marks the auto-provisioned assistant that will power the AI Insights sidebar; user-created assistants stay false.

ALTER TABLE assistants ADD COLUMN is_default boolean NOT NULL DEFAULT false;
CREATE UNIQUE INDEX CONCURRENTLY assistants_project_id_default_key
  ON assistants (project_id) WHERE (is_default AND deleted IS FALSE);

Safety

• Additive only: NOT NULL DEFAULT false backfills existing rows in-place; no null window.
• Index created CONCURRENTLY (-- atlas:txmode none) — no write lock on assistants.
• Soft-deleted rows excluded from the partial index so a replaced default never collides with its tombstone.

Validation

• mise lint:migrations: concurrent-index txmode ✅, ordering ✅
• atlas migrate lint (against a clean pgvector dev DB): no findings ✅

Stack

1. feat(dashboard): re-target AI Insights sidebar at project toolsets with RBAC #3123 — AI Insights → project toolsets + RBAC tokens (base)
2. this PR — is_default migration
3. next — default-assistant provisioning + resolver
4. next — dashboard ingress source kind + send endpoint
5. next — egress platform tool + browser read path
6. next — sidebar cutover

Ref: AGE-2631

---

Summary by cubic

Adds an is_default boolean to assistants and a partial unique index (excluding soft-deleted rows) to enforce at most one default assistant per project. Migration only; supports AGE-2631 by marking the auto-provisioned assistant for the AI Insights sidebar.

^{Written for commit 78970a8. Summary will update on new commits.}

[linear-code]

AGE-2631

[changeset-bot]

🦋 Changeset detected

Latest commit: 78970a8

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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Project	Deployment	Actions	Updated (UTC)
gram-docs-redirect	Ready	Preview, Comment	Jun 1, 2026 12:46am

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}