feat: add /project-costs Claude command and make target

[andre-salvati]

Summary

• Adds `scripts/project_costs.py` — queries AWS Cost Explorer (daily, last 30 days) and Databricks `system.billing.usage` via the SDK, printing two formatted tables
• Adds `make project-costs` Makefile target as a quick runner
• Adds `.claude/commands/project-costs.md` — `/project-costs` slash command that fetches the data then has Claude analyze it for anomalies, spikes, period comparisons, and cross-cloud correlation (S3/egress vs DBU spend)
• Updates `.gitignore` to track `.claude/commands/` so project-level slash commands are shared with the team while personal settings/hooks remain local
• Hardens `project_costs.py` against credential leakage and runtime failures: catches `FileNotFoundError` when AWS CLI is absent, adds `StatementState.CLOSED` to poll-exit set, guards `stmt.status.error` before `.message` on CANCELED, wraps `json.loads` against pager output, replaces raw AWS stderr (which can contain caller ARNs) with a sanitized message, prints `type(e).name` instead of full SDK exceptions (which can embed the workspace URL), and validates `--days >= 1`

Test plan

• `make project-costs` prints both tables (AWS daily + Databricks DBU by SKU)
• `/project-costs` in Claude Code runs the script and produces a written analysis with anomaly/spike/trend commentary
• AWS session expired → script prints sanitized error and continues to Databricks section
• Databricks free-tier workspace → script prints fallback message instead of crashing
• Code review findings addressed (credential leakage, infinite poll loop, null deref on cancel, JSON decode crash, negative --days)

🤖 Generated with Claude Code