Default new realms to a Workspace index card

[lukemelia]

What

create-realm.ts seeds every new realm's index.json. Point it at Workspace instead of CardsGrid, so a newly created realm opens to the Workspace experience. This is the "replace CardsGrid" keystone — the card, the recognition, and the migration are only meaningful once the default flips.

CardsGrid stays available for existing/bespoke realms; only the default for freshly created realms changes.

Depends on

• The Workspace card must exist (so a new realm's index resolves).
• The publish handler + prerender fast-path must recognize Workspace (so a new realm still gets the prerendered default-index treatment on publish, and the realm-server tests that assumed a CardsGrid default stay correct). This PR is stacked on that recognition branch.

Blast radius (audited)

The realm-server test helper createRealm seeds its own fileSystem, so it does not go through this handler — most realm-server tests are unaffected. Of the tests that hit the real /_create-realm endpoint:

• index-responses and realm-lifecycle seed their own index.json → unaffected.
• realm-routing's deadlock guard uses a mock fixture, not a real create+prerender → unaffected (comment now slightly historical).
• publish-unpublish creates its source realm via the endpoint and did rely on the default being CardsGrid → updated here: the default-index case now exercises Workspace, and the explicit case is repurposed to cover the legacy CardsGrid path, so both recognized defaults stay tested; the bespoke-index negative case is unchanged.

Also fixed software-factory comments that described the old CardsGrid default (the factory overwrites the index regardless, so it's behavior-neutral).

Test plan

realm-server ember-tsc, eslint, prettier clean. Not run locally — the realm-server publish/create suites need test-pg + their own prerender, which isn't wired up in this session; they run in CI. Given the default flip is behaviorally significant, CI's full realm-server + matrix suites (which create realms via /_create-realm) are the real confirmation. The publish test changes mirror the existing passing tests' structure.

🤖 Generated with Claude Code

[github-actions]

Host Test Results

    1 files  ±0      1 suites  ±0   2h 37m 44s ⏱️ +20s
3 602 tests ±0  3 587 ✅ ±0  15 💤 ±0  0 ❌ ±0 
3 621 runs  ±0  3 606 ✅ ±0  15 💤 ±0  0 ❌ ±0 

Results for commit 4b29bc8. ± Comparison against earlier commit b07a65f.

Realm Server Test Results

    1 files  ±0      1 suites  ±0   12m 1s ⏱️ +9s
1 920 tests ±0  1 920 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 999 runs  ±0  1 999 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 4b29bc8. ± Comparison against earlier commit b07a65f.