Fix inbound file handling and add cross-container file exchange

[lundog]

Why

Makes SimpleX file transfer reliable in external mode, including the common case where the simplex-chat runtime runs in a container (a filesystem OpenClaw doesn't share) while OpenClaw runs on the host or in its own container. It's transport-agnostic — no native-mode code — and everything is additive and backward compatible (unset config = today's behavior).

This is the file-exchange half of the earlier native-mode PR, split out per that PR's discussion so it can be reviewed and land on its own. A native-transport PR follows, stacked on top of this branch.

What

Inbound

• Fix a file-receive race: queue the pending file and retry the accept on rcvFileDescrReady, cancelling on timeout — a not-yet-ready XFTP descriptor no longer drops the file.
• Stage received files into OpenClaw's media/inbound store so media tools and sandboxed workspaces can read them.
• Resolve relative inbound paths against a new connection.filesFolder (default ~/.simplex/files, matching the bundled runtime service). When the runtime runs with --files-folder it reports a bare file name, which is joined to filesFolder; without it, the runtime reports absolute paths and they're read directly.
• Log the reported inbound path (absolute vs. relative) for diagnosability.

Outbound

• Add connection.outboundFolder. When set, outbound media is staged there before sending, and the staged path — valid on the runtime's side — is what travels over the WebSocket. Staged files are cleaned up after the send. Unset = the local path is passed as-is (single-filesystem default).
• Add connection.outboundFolderOnClient for split-path deployments where the two sides mount the shared directory at different paths: the plugin stages into outboundFolder but rewrites the directory prefix to this before sending, so no verbatim/matching path is required. No effect without outboundFolder.
• Resolve media://<subdir>/<id> store references to a physical path on send (e.g. re-sending a received file).

Housekeeping

• Bump ws to ^8.21.0 (dependency-audit fix).

Config connection gains filesFolder and outboundFolder (types, Zod schema, regenerated manifest). Adds unit tests for inbound resolution, outbound staging/cleanup, the readable-path guard, and media reference handling; documents both settings in the config reference and the runtime-setup guide (symmetric inbound/outbound sections).

Compatibility

Additive and backward compatible: existing configs are unaffected, and with filesFolder/outboundFolder unset the plugin behaves exactly as before.

Testing

Validated end-to-end against a containerized simplex-chat runtime with OpenClaw on the host, sharing an inbound volume and a verbatim outbound mount:

• Inbound: received files from a mobile client — reported name-only, resolved against filesFolder, staged into media/inbound.
• Outbound: the bot sent files sourced three ways — a local path, a media:// store reference, and an http(s) URL — each staged into the shared outbound dir, delivered, and cleaned up.
• tsc, biome, and vitest all green.