feat: add provider-backed workspace deletion

[steipete-oai]

Summary

• make delete the explicit lifecycle action for provider-backed workspaces across the Fleet UI, API, Go CLI, and SSH gateway
• preserve stop as a compatibility alias and retain Stop wording for legacy sessions
• reconcile pending creates before destructive work, fence explicit workspace-ID conflicts, and keep ambiguous adapter operations pending until exact ownership is proven
• clarify that ending a GitHub Actions-backed terminal session does not cancel the underlying workflow

Why

The existing stop path could finalize Crabfleet state without deleting the backing provider workspace. This adds an honest provider-backed Delete lifecycle while preserving compatibility for legacy and non-provider sessions.

Validation

• pnpm test (148/148)
• go test -race ./...
• go vet ./...
• TypeScript syntax and JSX bundle checks
• git diff --cached --check
• autoreview clean with no accepted/actionable findings

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: feat: add provider-backed workspace deletion This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.