feat(cli): path resolution relative to KDL file

[pesap]

Summary

Resolve data file paths (from="...") relative to the input KDL file location, with override support via CLI flag and environment variable.

Parent: #176

Acceptance Criteria

• from="./data/file.csv" resolves relative to the KDL file, not CWD
• --data-dir=<path> CLI flag overrides (highest priority)
• ARCO_DATA_DIR env var as fallback (middle priority)
• Default: relative to KDL file (lowest priority)
• Help text documents resolution order
• Test: relative path resolves from KDL location
• Test: --data-dir overrides relative path
• Test: ARCO_DATA_DIR used when no CLI flag
• Test: CLI flag beats env var

Implementation Notes

Locations:

• crates/arco-cli/src/config.rs — add --data-dir flag
• crates/arco-kdl/src/ — pass base path to parser/resolver

Resolution order:

1. --data-dir (CLI)
2. ARCO_DATA_DIR (env)
3. Parent directory of input KDL file

• HITL — affects user workflows, needs manual verification