A-1414: support hosted cache

[zhming0]

Description

Support hosted cache via Namespace artifact storage, driven by an nsc://<namespace> cache store URL under the agent_managed store type.

What this does:

• Scheme dispatch: NewBlobStore routes agent_managed URLs by scheme — nsc:// → NscStore, otherwise S3.
• Namespace from URL: the namespace is parsed from nsc://<namespace> and passed as --namespace to the nsc CLI (errors if the namespace is missing).
• Validation: validateCacheStore now accepts s3://, nsc://, and file:// for agent_managed, and rejects unknown schemes.
• Default expiry: uploads pass --expires_in 24h (temporary stopgap; TTL refresh-on-access parity with S3 tracked in A-1437).
• Context cancellation: the nsc CLI now runs via exec.CommandContext, so uploads/downloads abort on context cancel/timeout.
• Cleanup: removed the unused local_hosted_agents store type (NSC is now reachable only via nsc://).

Tests are lean and don't require the nsc binary: URL parsing, namespace passthrough via an injected command-runner seam, validation short-circuiting, and nsc:// dispatch. The existing NSC_INTEGRATION_TEST-gated end-to-end test is retained.

Follow-ups: A-1437 (TTL refresh, pending upstream), A-1438 (cache busting / invalidation).

Context

A-1414

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

LLM and me.

[buildsworth-bk-app]

On it! Follow along with my review here: https://buildkite.com/buildkite/buildsworth-review-pull-request/builds/1215

[buildsworth-bk-app]

I read this as L2 rather than an approval because it changes cache-store dispatch/validation in the agent-managed path. I found one non-blocking mismatch around the local-testing URL path.

Want to dig deeper? The full session log is attached to this Buildkite build. Download the session file and open a new pi session with it:

Download the buildsworth logs from build 1215, then answer my questions about the findings.