fix(cli): reload MCP config after project selection

[luantaraschi]

Recreated from #966, which GitHub auto-closed when this repository's history was rewritten. Same change, rebased onto the new main at d4902003. Fixes #957.

Picking a different project through the project picker changes the working directory, moves the project root and resets the client, but never re-scans .agents/ for the new directory. cachedAgentsDir and cachedAgentsByMode in local-agent-registry.ts go on serving the launch directory, so the selected project's MCP servers and local agents never reach the base agent definition.

handleProjectChange in cli/src/index.tsx had the chdir, setProjectRoot and resetCodebuffClient inline. This moves that sequence into activateProject() in project-picker.ts and puts the registry reload in the middle of it, so the ordering lives in one place instead of being duplicated at the call site.

reloadLocalAgentRegistry() clears only cachedAgentsDir and cachedAgentsByMode, and that is deliberate. The module keeps five mutable caches. The other three, userAgentsCache, userAgentFilePaths and mcpServersCache, are reassigned by initializeAgentRegistry() on both its success and failure paths, mcpServersCache from loadMCPConfigSync(), which reads the cwd. So getLoadedMCPServers() is covered as long as the chdir happens first. The two derived ones are the only ones nothing else resets.

The test writes real .agents/*.ts and mcp.json files into temporary launch and project directories, warms the caches in the launch project, switches through activateProject(), then asserts the selected project supplies the agent directory, the local agent list, the MCP server and the base agent definition, with the launch project's agent gone. A second case covers the { reloadAgentRegistry: false } skip path, where the chdir and project root still move but the launch project's agent is still the one served. I checked that case earns its place by making activateProject reload unconditionally: it is the only test that then fails.

Validation on this head:

bun test cli/src/__tests__/utils/     23 pass, 0 fail
cd cli && bun run typecheck           10 errors

Those 10 are the same 10 a clean checkout of main produces, all react-dom/server declarations in component tests this change does not touch. Counted both ways to be sure.