feat: support project-scoped Langfuse configuration

[suolongcn]

Closes #40.

Pi sessions can now target a workspace-specific Langfuse instance through .pi/langfuse.json in the current working directory. The plugin selects that file before the global agent config, while environment variables retain their existing precedence.

Missing, unreadable, malformed, or non-object project files fall back to the global config. Valid project objects are used as a whole, including empty or incomplete objects, so credentials and endpoints from different instances are not merged. The README documents precedence, fallback behavior, and keeping project credentials out of version control.

Validation:

• TypeScript typecheck passed (tsc --noEmit).
• Full test suite passed: 141 tests, 0 failures (Node 24.21.0, pi 0.84.3, Ubuntu/WSL).
• Added configuration regressions and a real Pi integration test proving that only the project target receives a trace while the global target receives no requests.
• Verified a real Pi session with a deterministic mock model against a local Langfuse instance: the project config produced a Pi Turn trace containing 6 observations, including generations and tool calls. Test credentials were kept out of the commit.