tools: Install on first use instead of baking them into the image

Baked installs had two failure modes: anything installed into $HOME
at build time is shadowed by the container-home bind mount at runtime,
and system-wide installs are root-owned, so the tools' own update
commands (pi update, claude update, opencode upgrade) failed with an
unwritable install path. With pi releasing new versions almost daily,
the only remedy, a full image rebuild that also deleted every
container, was too heavy for that cadence.

Now each tool is installed at runtime into the persistent container
home by its official installer (no sudo), the same way the Docker
image itself is built on first use. Self-updates work inside the
container and survive image rebuilds and container recreation. The
image shrinks to a plain Arch base, and agent-container update no
longer removes containers, so sudo-installed project dependencies
survive it too.

docker exec now attaches a TTY only when stdin is one, so scripted
runs like 'pi -p' work without a terminal.

README.md

@@ -4,7 +4,10 @@ Run OpenCode, Claude Code, and Pi (pi.dev) inside a shared Arch Linux Docker con
 
 ## Features
 
-- Arch Linux-based image with OpenCode, Claude Code, and Pi pre-installed
+- Tools are not baked into the image: OpenCode, Claude Code, and Pi are
+  bootstrapped into the persistent container home on first use (official
+  installers, no sudo), so `pi update`, `claude update`, and
+  `opencode upgrade` work inside the container
 - Runs as the host user (same username, UID, GID)
 - **Per-project isolation**: Each project gets its own container (identified by project path hash)
 - **Shared persistent home**: All containers mount the same home directory from XDG_DATA_HOME, allowing tools and credentials to persist across projects
@@ -16,7 +19,7 @@ Run OpenCode, Claude Code, and Pi (pi.dev) inside a shared Arch Linux Docker con
 
 ## Install
 
-Clone the repository:
+On a fresh machine, two steps are all it takes. The agent tools themselves are not part of the image and need no installation step: on first use, agent-container.py automatically runs each tool's official installer inside the container (no sudo), just like it builds the Docker image automatically if it is missing.
 
 ```sh
 cd ~/Projects/
@@ -31,13 +34,29 @@ source ~/Projects/agent-container/agent.aliases
 
 This makes the `opencode`, `claude`, `pi`, and `agent-container` commands available in new sessions, plus the `opencode-local`, `claude-local`, and `pi-local` variants that run the tools directly on the host.
 
-Optionally run the setup script to share agent state (`.claude`, `.pi`, `.agents`, OpenCode state, ...) between the container and the host:
+That is the whole install. From any project directory, `opencode`, `claude`, and `pi` now work: the first run installs the tool into the persistent container home, where logins (via `/login`), settings, and history persist across projects, containers, and image rebuilds.
+
+### Optional: share agent state with the host
+
+By default, host and container keep separate state: the tools live entirely in the container home, and nothing in your real `$HOME` is moved or linked. If you also run the tools on the host (the `*-local` variants) and want shared logins, settings, and history, run the setup script once:
 
 ```sh
 ~/Projects/agent-container/setup.sh
 ```
 
-It shows a plan first and asks for confirmation before moving anything (`--dry-run` previews without changes, `--yes` skips the question). Existing state is moved into the container home and symlinked back into `$HOME`, so containerized tools and the `*-local` variants share logins, settings, and history. See "Sharing agent state between host and container via symlinks" below. The script is safe to re-run and skips items it cannot handle safely.
+It shows a plan first and asks for confirmation before moving anything (`--dry-run` previews without changes, `--yes` skips the question). Existing state is moved into the container home and symlinked back into `$HOME`. See "Sharing agent state between host and container via symlinks" below. The script is safe to re-run and skips items it cannot handle safely.
+
+### Optional extras
+
+```sh
+# Share your git config with the containers (hard link):
+ln ~/.gitconfig ~/.local/share/agent-container/container-home/.gitconfig
+
+# Desktop notifications when OpenCode needs attention:
+mkdir -p ~/.local/share/agent-container/container-home/.config/opencode/plugins
+cp plugins/opencode/osc99-notify.js \
+   ~/.local/share/agent-container/container-home/.config/opencode/plugins/
+```
 
 The container home directory at `$XDG_DATA_HOME/agent-container/container-home/` serves as a central `$HOME` inside every container, independent of which project directory you start in. Everything written to `$HOME` inside the container persists there.
 
@@ -77,13 +96,21 @@ The image is built automatically on first use if it does not already exist. The
 
 ### Updating
 
-To rebuild the image with the latest versions of OpenCode, Claude Code, and Pi:
+The tools are not part of the image: on first use, each tool's official installer runs inside the container (no sudo) and installs into the persistent container home. They update themselves there, and updates survive container recreation and image rebuilds:
+
+```sh
+agent-container pi update        # or run `pi update` inside a session
+agent-container claude update
+agent-container opencode upgrade
+```
+
+To rebuild the base image with a fresh Arch Linux userland:
 
 ```sh
 agent-container update
 ```
 
-This removes all existing containers and rebuilds the image from scratch. Containers are recreated automatically on the next run. The persistent home directory is not affected.
+This rebuilds the image with a freshly pulled base but keeps all existing containers and everything installed in them. The persistent home directory is not affected.
 
 ### Purge