fix(pi-bash-bg): stop replacing the bash tool so shellCommandPrefix keeps working

[mgabor3141]

Problem

Since 0.1.0, pi-bash-bg calls createBashTool(process.cwd()) and pi.registerTool(...) to append a background-job note to the bash tool description:

const bashTool = createBashTool(process.cwd());
pi.registerTool({
    ...bashTool,
    description: `${bashTool.description} ${BASH_DESCRIPTION_APPENDIX}`,
});

The freshly-constructed bashTool is created without the commandPrefix, shellPath, or spawnHook options pi wires onto its built-in bash tool. In pi's _refreshToolRegistry, extension-registered tools win over built-in ones with the same name, so that replacement silently drops those options.

Concretely, any user with something like this in ~/.pi/agent/settings.json:

{ "shellCommandPrefix": "export PI_ROOT=..." }

sees their prefix stop running as soon as pi-bash-bg is loaded. Same for shellPath. I hit this while setting PI_ROOT for agent bash sessions and traced it through getShellCommandPrefix -> createAllToolDefinitions({ bash: { commandPrefix } }) -> the registry override above.

Fix

Move the background-job guidance into the system prompt via a before_agent_start hook (same mechanism the extension already uses for other prompt tweaks), and stop registering a replacement bash tool. Pi's built-in bash stays put, so shellCommandPrefix, shellPath, and any spawnHook keep working.

The guidance section injected into the system prompt:

## Background jobs (\`command &\`)

The bash tool supports backgrounding processes with \`&\`.
Background process stdout/stderr is captured to a temp log file.
Each rewritten command reports \`[bg] pid=<PID> label=<LABEL> log=<PATH>\` in its output.
Use \`cat <PATH>\` to check output and \`kill <PID>\` to stop the process.

It's exported as BASH_BG_SYSTEM_PROMPT_SECTION and the handler no-ops if it's already present, so repeated before_agent_start events don't duplicate it.

Verification

• yarn test --run packages/bash-bg: 76/76 pass (new test covers that no tool is registered, handlers are wired up, and the system prompt is augmented once).
• yarn lint: clean.
• End-to-end: with this built locally and shellCommandPrefix set, env | grep ^PI_ROOT in agent bash now returns the expected value; before the fix it was empty.

Changeset

Included as a patch bump on pi-bash-bg.