feat(cli): auto-detect project root by walking up for Cargo.toml

[rocketman-code]

Summary

• Add find_project_root() that walks up from CWD looking for Cargo.toml, matching cargo's project discovery behavior
• Change --project flag on build and profile from defaulting to . to auto-detecting via find_project_root() (explicit --project still overrides)
• Update default_runs_dir(), default_tags_dir(), and find_latest_binary() to resolve paths relative to the discovered project root instead of CWD

All piano commands (build, run, profile, report, diff, tag) now work from any subdirectory within a Rust project.

Closes #135
Closes #85

Test plan

• 4 unit tests for find_project_root (project dir, subdirectory, nested subdirectory, error case)
• Integration test: piano build from src/ subdirectory without --project
• Integration test: piano report from src/ subdirectory without PIANO_RUNS_DIR
• Integration test: piano run from src/ subdirectory
• Existing test suite passes (all commands that pass --project explicitly still work)