feat: add inventory data view command

[ecv]

What

Adds datumctl inventory — a read view over the Datum Cloud physical inventory (providers, regions, sites, clusters, nodes) now loaded into the inventory.miloapis.com CRDs.

Generic datumctl get <kind> already lists these via the CRDs' additionalPrinterColumns. This command adds the topology navigation and rollups that get cannot express.

Commands

Command	Does
inventory providers|regions|sites|clusters|nodes	List per kind, curated columns, -o table/json/yaml
inventory tree [--region]	region → site → node hierarchy + per-region clusters
inventory summary	totals per kind, sites/nodes per region, sites per provider

Design

• Filters: --region/--site/--cluster resolve server-side via the topology.inventory.miloapis.com/* labels the platform propagates; --provider (and cluster --site) match the spec ref client-side (no topology label exists for those).
• Defaults to --platform-wide (inventory lives on the platform root); --organization/--project override.
• Reads via the dynamic client over unstructured objects — datumctl takes no dependency on the inventory Go types.

Tests

8 unit tests covering field extraction, row formatting, client-side filtering, tree, summary tally/union, and json/yaml rendering of unstructured lists. go test ./... green.

Verification

• Verified against prod (read-only, --platform-wide) — staging auth was down. All lists, filters, tree, summary, and json output confirmed against the real fleet (4 providers / 18 regions / 19 sites / 18 clusters / 90 nodes). See verification comment below.

Docs

docs/inventory.md added.

Closes #211

[ecv]

Verified against prod (read-only)

Staging auth is down, so validated against prod with --platform-wide. All commands read-only (List only).

Lists — real fleet rendered with correct columns + cross-refs + Ready:

• providers: 4 (aws, gcp, netactuate, vultr)
• regions: 18 (all Ready)
• sites: 19 (regionRef/providerRef/type populated)
• clusters: 18 (region label + controlPlaneSiteRef + role)
• nodes: 90 (siteRef/clusterRef/role/arch/cpu/phase)

Filters

• sites --region us-central-1 → 2 sites (server-side region label) ✓
• nodes --site us-central-1a → site label filter ✓
• nodes --cluster us-central-1-lab → cluster label filter ✓
• sites --provider gcp → empty (client-side providerRef; all sites are netactuate) ✓

tree --region us-central-1 → region → clusters line → sites → nodes ✓

summary → totals (4/18/19/18/90), sites+nodes per region, sites per provider. Correctly shows us-central-2: 1 site / 0 nodes (Chicago ISO_ID site, no edge nodes loaded) ✓

json → full unstructured objects render ✓

Functionally complete and confirmed against real data. Left as draft pending review.

[scotwells]

@ecv we shouldn't bake this directly into datumctl. Let's make it a plugin and have the CLI live in the inventory service.

[ecv]

I knew I was forgetting something you said at the bar!

[ecv]

Superseded by milo-os/inventory#41. Per @scotwells' review, this shouldn't bake into datumctl core — it's now a datumctl plugin (datumctl inventory ...) living in the inventory service, using its own typed API. Closing; no datumctl core changes needed (the plugin SDK already covers everything).