Harden marketplace task creation

[signerless]

Summary

• Add approved marketplace task templates plus the safe template tool flow: list, get, create from template, and submit proposal.
• Gate raw agenc.createTask off by default for agent routing while keeping explicit CLI and webchat paths opt-in.
• Require approval for all raw agenc.createTask calls and remove the LLM argument repair path for raw task creation.

Why

Issue #350 identified prompt-injection risk in raw marketplace task creation: arbitrary user-provided task fields could be treated as instructions, repair logic could normalize loose calls into valid tasks, and approvals only triggered above a reward threshold. This PR shifts agent-driven creation to approved templates and preserves raw creation only for explicit trusted entry points.

Validation

• npm exec tsc -- --noEmit from runtime/
• npm exec vitest run src/marketplace/approved-task-templates.test.ts src/tools/agenc/tools-task-templates.test.ts src/gateway/approvals.test.ts src/gateway/system-prompt-routing.test.ts src/llm/chat-executor-tool-utils.test.ts
• git diff --check

[signerless]

Updated this PR with claim-time marketplace job spec verification and the main audit roadmap note.

Cross-repo PRs:

• Protocol: Harden task claims on job spec pointers agenc-protocol#26
• SDK: Expose task job spec claim helpers agenc-sdk#32

Additional validation:

• AGENC_PROTOCOL_WORKSPACE_ROOT=/Users/tetsuoarena/agenc-protocol-prprep npx vitest run src/task/operations.test.ts src/cli/index.test.ts src/tools/agenc/mutation-tools.test.ts tests/marketplace-cli.integration.test.ts --reporter dot
• npx tsc --noEmit
• git diff --check