fix(cli): display timestamps in local time instead of UTC

[benoitf]

Summary

• Display sandbox and provider timestamps in the user's local timezone instead of UTC, fixing a visual drift where creation times appeared hours in the past for non-UTC users
• Use ISO 8601 with timezone offset for JSON output (--output json)
• Apply the same fix to the TUI timestamp display

Related Issue

Closes #2106

Changes

• crates/openshell-cli/src/run.rs: Replace format_epoch_ms manual UTC arithmetic with chrono::Local for YYYY-MM-DD HH:MM:SS local-time display. Add format_epoch_ms_iso8601 for JSON output (2026-07-01T16:56:41+02:00). Replace format_timestamp_ms (audit log) similarly. Remove civil_from_days helper. Update JSON call sites in sandbox_to_json and provider_to_json to use ISO 8601.
• crates/openshell-tui/src/lib.rs: Replace format_timestamp with chrono::Local. Remove days_to_ymd helper. format_age (relative time) is unchanged — it was already correct.
• crates/openshell-tui/Cargo.toml: Add chrono = "0.4" dependency.

Testing

• mise run pre-commit passes (markdown lint failures are pre-existing untracked files)
• All 185 CLI unit tests pass, including 3 updated timestamp tests
• TUI compiles cleanly
• Manual: openshell sandbox list — CREATED column matches local clock
• Manual: openshell sandbox list --output json — created_at uses ISO 8601 with offset

Checklist

• Follows Conventional Commits format
• Signed off for DCO compliance
• No AI attribution in commits

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