feat(sdk/workspace): Capabilities + CapabilityReporter

[lIang70]

Summary

Prep for P4 — sdkx/retrieval/workspace.

Adds an optional interface that `Workspace` implementations satisfy to publish their storage characteristics (atomic rename, read-after-write, write durability, distributed access). Higher-layer adapters that need accurate semantics — e.g. the upcoming LSM-style retrieval Index that relies on atomic Rename for manifest swaps — read these via `CapabilitiesOf` instead of hard-coding per-impl assumptions.

Impl	AtomicRename	RaW	Durable	Distributed
`MemWorkspace`	✅	✅	❌	❌
`LocalWorkspace`	✅	✅	✅	❌
`ScopedWorkspace`	(forwards inner)	(forwards inner)	(forwards inner)	(forwards inner)
`ScopedGitWorkspace`	inherits via embedding

Contract: zero value = no guarantees. Adapters that don't bother to type-assert `CapabilityReporter`, or that handle a workspace whose author hasn't implemented it, get the safe path. Adding a new field is therefore additive.

The sdkx-side `objstore.objectWorkspace` will land its `Capabilities()` method on the follow-up sdkx PR that pins the resulting sdk tag.

Test plan

• `go test ./workspace/... -count=1` passes
• Cases: each impl reports the documented caps; `Scoped` forwards a reporter inner; `Scoped` over a non-reporter inner yields zero value; `CapabilitiesOf(nil)` yields zero value; non-reporter `Workspace` yields zero value
• No behavioural changes for existing callers — purely additive interface

Notes

• No `[skip-tag:sdk]`. The follow-up sdkx PR will pin this tag immediately, so accumulating with another sdk change would block downstream.
• Capabilities axis kept deliberately small (4 booleans). Future feature-bearing capabilities (Watch / Snapshot / PresignURL) should be added as sub-interfaces (the same pattern `CapabilityReporter` itself follows), not as bool flags here.

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