fix: update built-in prompt work commands

[KalleBylin]

Summary

Going through some of the docs and prompts I found commands that seem to have been replaced in recent changes to the repo. This PR is created to reduce the drift:

• update the built-in worker prompts to use the current hook/sling workflow instead of removed gc agent claim/claimed commands
• fix the pool-worker prompt to use the shared pool target and execute pooled work directly
• tighten prompt regression tests by rendering prompts and deriving command names from the live CLI definitions
• align gc hook injection copy with the assign/route terminology used by the prompts

Testing

• go test ./cmd/gc -run 'TestMaterializeBuiltinPrompts|TestMaterializeBuiltinPromptsOverwrites|TestMaterializeBuiltinFixedWorkerPromptsUseCurrentHookCommand|TestMaterializeBuiltinLoopPromptsUseCurrentHookAndSlingCommands|TestRenderBuiltinPoolWorkerPromptUsesPoolTemplateTarget|TestHook.|TestRenderPrompt.'
• make check (fails the same way on a clean local origin/main checkout; likely macOS-specific issues involving /var vs /private/var path canonicalization, Darwin unix socket path limits, and shell portability in mail-provider tests rather than this PR diff)

[KalleBylin]

Converting to draft and will review if still needed or close like with #12