v0.7.1 - Execution root notice

Fix: a sandboxed run is spawned with cwd=<worktree>, but agents did their recon in the main repo. The only absolute path an agent held was context_handoff_dir, which anchors at the trace root (the shared main repo) so the trace lands in the shared sssf.db. Nothing told the agent where its code root was, so it generalised from the handoff path and planned against the wrong branch. cwd does not help: an agent that writes an absolute path never consults it.

agents._run_instance now appends an # Execution root block, rendered off run.repo_root, to every agent's system prompt - after the project guidance. Unconditional, so it covers every agent, every ADW and all three backends (claude_code, pi, cursor) with no prompt-template change. context_handoff_dir is carved out by name so read-only agents (writes: []) keep using it verbatim as their write target.

agents.py and the two operator-skill docs are MANAGED, so stamped repos pick this up on their next update - no re-stamp needed.

PR #43.