Add MatrixClaw subagents

[Suren878]

Summary

• Add delegate_task for hidden MatrixClaw child subagent sessions with restricted child tools and stored task audit links.
• Add external subagent runtimes for Codex and Claude Code, with model/runtime visibility in the parent system prompt.
• Add TUI rendering, session/runtime integration, README docs, migration coverage, and version bump to 0.1.12.

Test Plan

• go vet ./...
• go test ./...
• go build -o bin/matrixclaw ./cmd/matrixclaw
• go build -o bin/matrixclawd ./cmd/matrixclawd
• /root/go/bin/golangci-lint run --new-from-rev=origin/main
• live delegate_task smoke tests for matrixclaw, codex, and claude