feat(config): create a new project's memory file from Settings

The Memory tab could only show this machine's user memory and the
dashboard's own project file. Setting up a project you had not run
through the dashboard meant opening a terminal — `PUT /file`'s
"project" scope always resolves against the server's cwd, on purpose.

Add one narrow route per tool that writes into the directory you name:
CLAUDE.md for Claude Code, AGENTS.md for Codex, both through the same
`project-memory.js`. Being the only write path that honours a
caller-supplied directory, it pays for that:

  - the directory must be absolute and already exist — nothing is
    created for you;
  - the filename comes from the route, never from the body;
  - an existing file, symlink included, is a 409 — this can only
    create, never overwrite;
  - it sits behind the loopback-Origin guard /api/run uses, so a page
    in the user's browser cannot plant agent instructions in a project
    directory. That guard moves to lib/same-origin.js to be shared.

The modal takes the dashboard's known project dirs as suggestions on a
free-text input rather than a closed list: a project it has never seen
is exactly the case this exists for.

Mutation-checked: dropping the origin guard, and swapping lstat for
stat, each fail a test.

client/src/i18n/locales/en/codexConfig.json

@@ -56,6 +56,20 @@
   "memory": {
     "missing": "No AGENTS.md found."
   },
+  "projectMemory": {
+    "newButton": "New project memory",
+    "title": "Create {{file}} for another project",
+    "dirLabel": "Project directory",
+    "dirHelp": "Absolute path to a project that already exists on this machine. {{file}} is created inside it; an existing one is never overwritten.",
+    "dirRequired": "A project directory is required",
+    "createButton": "Create",
+    "created": "Created {{path}}",
+    "template": "# Project memory\n\nWhat this project is, and anything an agent should know before touching it.\n\n## Stack\n\n## Conventions\n\n## Commands\n",
+    "contentLabel": "Content",
+    "cancel": "Cancel",
+    "saving": "Creating…",
+    "close": "Close"
+  },
   "config": {
     "missing": "No config.toml found."
   },