fix(mtmharness): harden alpha5 coding runtime

[codeh007]

Summary

• keep Modern Go caches inside workspace-write workspaces
• restore /ponytail empty-input behavior to the configured full mode
• validate packaged prompt metadata and client behavior

Verification

• pnpm install --frozen-lockfile
• pnpm --filter mtmharness run typecheck
• pnpm --filter mtmharness run test
• npm pack --dry-run --json
• DSH 0.1.2-alpha.5 isolated plugin install

Closes codeh007/gomtm#816 follow-up

[coderabbitai]

Important

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Review profile: CHILL

Plan: Team

Run ID: 377d3092-cb51-4146-8762-2040f7208166

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