INST-1 · isCore / namespaceType flag isolating main

> Item derived from the fact-checking cycle analysis (**2026-07-03**). Production numbers (namespace `main`), unless noted.

### Finding (evidence)
57% of 2024 claims (68 of 119) are `test-environment` (31) + workshop (37), with a ~0 h cycle. This alone made "2024 look 3× faster" (3.2 h vs. 6.6 h for `main` only) and skewed the entire historical median.

### Action
Derive `isCore` (= `nameSpace === "main"`) and `namespaceType` (core / workshop / test / event) from the existing `nameSpace`. Filter `isCore: true` by default in any cadence metric.

### Impact
Fixes the distortion across **all** metrics at the lowest effort. No data deleted — workshops remain queryable by category.

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- **Priority:** 🔴 High · **Effort:** S (small)
- **Dependencies:** none — immediate item, unblocks the rest.
- **Epic:** #2489