muxr v3.6.0 Config seam: muxr natively discovers per-repo muxr.toml fragments and carries an open [repos.<name>.ext] namespace it hands to extensions, so estate and preference data is drop-in config, never a muxr rebuild, and a repo absent on a machine is simply not discovered (zero cross-machine knowledge). Adds `muxr config` for extensions to read the merged view, and a config-tunable [readiness].stale_busy_secs. Defaults byte-identical to 3.5.2. ADR 0005.