feat: turn session recording into a plugin (B13)

plugins/session-recording/README.md

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+# Session Recording
+
+Records terminal sessions and lets you play them back or download them.
+
+Bundled with Termix and enabled by default. It imports only
+`@termix/plugin-sdk`.
+
+## What it does
+
+- **Recording**: an SSH terminal session is recorded as an asciicast, written
+  incrementally in 300ms batches as it happens (issue #1049).
+- **Playback**: view a recording inline, scrub through it, copy its plain
+  text, or download it as a recording file or plain text.
+- **Retention**: an admin setting prunes recordings older than a configured
+  number of days, checked at startup and every 24 hours.
+- **Per-host switch**: recording can be turned off for a host.
+- **`recordings.writer`**: a service other plugins consume. `open(meta)`
+  starts an incremental recording (ssh-terminal); `createFinished(input)`
+  inserts a row for a recording the caller already wrote to disk itself
+  (remote desktop's guacd recordings).
+
+Recordings outlive the account they were made under: deleting a user
+anonymizes their recordings rather than deleting them, since a recording is
+evidence about the host as much as about the person.