feat(sigma): add Sigma Computing context-source adapter (#316)

* feat(sigma): add Sigma Computing context-source adapter

Closes #168

Adds a full ingest adapter for Sigma Computing so `ktx ingest` can pull
data model specs and workbook summaries into the ktx context layer. The
implementation follows the same fetch → chunk → project → LLM pattern
used by the Looker, Metabase, and MetricFlow adapters.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(sigma): address PR review comments

- Remove manifest from rawFiles; moves to peerFileIndex so fetchedAt
  changes don't mark all work units dirty every run
- Fix workbookFilter.updatedSince eviction bug: fetch full universe first,
  apply filter client-side, evict only on archived/deleted
- Remove measure projection entirely; project() writes measures: [] and
  the sigma_ingest skill surfaces Lookup/aggregation formulas as wiki prose
- Remove joins projection (v1 limitation); project() writes joins: [] and
  Lookup relationships are described in wiki prose instead
- Remove write-back dead code: createDataModel, updateDataModel,
  SigmaDataModelPushResult, mutate/post/put
- Fix emitBatches notes pluralization bug ('2 data modelss' → '2 data models')
- Add tokenInflight dedup on ensureToken to coalesce concurrent auth requests
- Retry spec fetch when existing staged spec is null (transient failure cache)
- Drop unused WorkbookFilter import from client-port.ts
- Note in docs that joins are not projected from Sigma data models in this release

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* updates

* fix(sigma): restore sigma in local adapter test + small cleanups

The gdrive↔sigma merge dropped 'sigma' from the expected adapter source
list in local-adapters.test.ts while keeping gdrive, so the slow TS suite
failed even though the source registers both. Add 'sigma' back at its
registration position (after metabase, before gdrive).

Also:
- Move the orphaned SigmaPullConfig docstring onto the schema it documents
  and drop the stale BullMQ reference (standalone ktx has no BullMQ; the
  config lives in the ingest job's bundleRef.config).
- Drop an O(n^2) find() round-trip in fetch() when building the active
  data-model list; filter once and reuse for the eviction id set.

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Andrey Avtomonov <andreybavt@gmail.com>
Co-authored-by: Luca Martial <48870843+luca-martial@users.noreply.github.com>

docs-site/content/docs/configuration/ktx-yaml.mdx

@@ -119,6 +119,7 @@ context-source drivers share the map.
 | `dbt` | Context source | `driver`, one of `source_dir` or `repo_url` | `branch`, `path`, `profiles_path`, `target`, `project_name` |
 | `metricflow` | Context source | `driver`, `metricflow.repoUrl` | `metricflow.branch`, `metricflow.path`, `metricflow.auth_token_ref` |
 | `notion` | Context source | `driver`, `auth_token_ref` | `crawl_mode`, `root_*_ids`, `max_*_per_run` |
+| `sigma` | Context source | `driver`, `client_id`, `client_secret_ref` | `api_url` |
 
 ### Warehouse drivers
 
@@ -345,6 +346,31 @@ connections:
 | `max_knowledge_creates_per_run` | Max new wiki pages created per run (0-25). |
 | `max_knowledge_updates_per_run` | Max existing wiki pages updated per run (0-100). |
 
+### Sigma
+
+```yaml
+connections:
+  sigma-main:
+    driver: sigma
+    api_url: https://api.sigmacomputing.com
+    client_id: "<your-client-id>"
+    client_secret_ref: env:SIGMA_CLIENT_SECRET
+    workbookFilter:
+      includeArchived: false
+      includeExplorations: false
+      updatedSince: "2026-01-01T00:00:00Z"
+```
+
+| Field | Purpose |
+|-------|---------|
+| `api_url` | Sigma API base URL. Defaults to `https://api.sigmacomputing.com` (GCP US). Override for AWS US (`https://aws-api.sigmacomputing.com`) or other regions. |
+| `client_id` | Sigma OAuth client ID. Required. |
+| `client_secret` / `client_secret_ref` | Literal secret or reference. Prefer the `_ref`. |
+| `connectionMappings` | Maps Sigma internal connection UUIDs to **ktx** warehouse connection IDs. Enables `sl_validate` for projected semantic-layer sources. |
+| `workbookFilter.includeArchived` | Include archived workbooks during ingest. Default: `false`. |
+| `workbookFilter.includeExplorations` | Include exploration workbooks during ingest. Default: `false`. |
+| `workbookFilter.updatedSince` | ISO 8601 date string. Only workbooks updated on or after this date are fetched. Useful for limiting ingest scope at large scale. |
+
 ## `setup`
 
 Captured by the setup wizard. The only field **ktx** still reads is