CLI: Add --dry-run flag for mutating commands

[titouanmathis]

Context

Following agent-friendly CLI best practices, agents should be able to preview what a command would do before committing. This is especially important for destructive actions.

Proposal

Add --dry-run support to all mutating commands:

• Create: time add, tasks add, comments add, deals add, bookings add, pages add, discussions add, companies add
• Update: all update subcommands
• Delete: time delete, pages delete, discussions delete, attachments delete

Behavior

$ productive time add --service 123 --time 480 --note "Dev work" --dry-run
Would create time entry:
  service: 123
  time: 480 (8h 0m)
  date: 2026-03-24
  note: Dev work
No changes made.

$ productive time delete 12345 --dry-run
Would delete time entry 12345
No changes made.

With --format json:

{"dry_run": true, "action": "create", "resource": "time_entries", "payload": {...}}

Implementation

• Validate all inputs as normal (fail fast on missing required fields)
• Build the request payload but skip the API call
• Display what would happen
• Exit with code 0