doc(persist-claude-mem): add README

Explains volume naming, symlink behavior, backup on collision,
and host isolation from the claude-mem MCP server process.

Author: ckagerer

src/persist-claude-mem/README.md

@@ -0,0 +1,31 @@
+# Persist claude-mem knowledge base (persist-claude-mem)
+
+Mounts a named Docker volume at `~/.claude-mem` so the [claude-mem](https://github.com/NicolasMassart/claude-mem) knowledge base survives container rebuilds without conflicting with the host claude-mem MCP server process.
+
+## Example Usage
+
+```json
+"features": {
+    "ghcr.io/ckagerer/devcontainer-features/persist-claude-mem:0": {}
+}
+```
+
+## How It Works
+
+During build, the feature mounts a named Docker volume at `/.persist-claude-mem`. On first container start, `postCreateCommand` runs an init script that symlinks `~/.claude-mem -> /.persist-claude-mem`.
+
+Volume name pattern:
+
+```
+devcontainer-<workspaceFolderBasename>-persist-claude-mem-<devcontainerId>
+```
+
+Each workspace gets its own volume, so knowledge bases from different projects stay isolated.
+
+If `~/.claude-mem` already exists as a real directory when the container starts, the init script backs it up with a timestamp (`~/.claude-mem.pre-persist.<epoch>`) before creating the symlink.
+
+## Notes
+
+### Host isolation
+
+The volume mounts at `/.persist-claude-mem`, not directly at `~/.claude-mem`. This keeps the container's knowledge base on a separate filesystem path from the host's `~/.claude-mem`, so the host claude-mem MCP server process never reads from or writes to the container volume.