piano report only finds project-local data when run from project root

[rocketman-code]

Context

PR #77 added project-local run data at target/piano/runs/. The default_runs_dir() function probes for this directory using a relative path: PathBuf::from("target/piano/runs").

Problem

This means piano report only finds project-local data when invoked from the project root directory. Running cd src && piano report or invoking from a subdirectory falls through to the global ~/.piano/runs/ fallback.

This is consistent with how Cargo works (you typically run cargo from the project root), but could be surprising to users.

Options

1. Walk up parent directories looking for target/piano/runs/, similar to how Cargo finds Cargo.toml
2. Document the assumption that piano commands should be run from the project root
3. Accept as-is (consistent with Cargo behavior)

Discovered during code review of PR #77.