fix(cli): don't hang on stdin for destructive ops in non-interactive sessions

[PeterGuy326]

Problem

Destructive compat commands (e.g. calendar room delete, calendar participant delete) unconditionally read a yes/no confirmation from stdin when --yes is not passed. In a non-interactive session (pipe / CI / test harness / AI agent), stdin hits EOF immediately, so the command:

• prints the confirmation prompt + Operation cancelled to stderr
• leaves stdout empty (no JSON)
• returns exit code 0

The destructive operation silently does not run, yet reports success. Any -f json caller parsing the empty stdout treats it as a failure. This is the root cause of the calendar failures in the cli_to_mcp evaluation.

Reproduce (before)

printf '' | dws calendar room delete --event FAKE --rooms FAKE -f json ; echo "exit=$?"
# stderr: "⚠️  This is a destructive operation. Confirm? (yes/no): Operation cancelled"
# stdout: (empty)
# exit=0

Fix

Before falling into the interactive prompt, detect a non-interactive stdin (os.Stdin is not a character device) and return a structured validation error instructing the caller to pass --yes, instead of hanging on / EOF-ing the prompt.

After

printf '' | dws calendar room delete --event FAKE --rooms FAKE -f json ; echo "exit=$?"
# structured JSON validation error: "...re-run with --yes to confirm..."
# exit=non-zero

printf '' | dws calendar room delete --event FAKE --rooms FAKE --yes -f json
# bypasses the gate, reaches real execution (business API error for the fake event)

Interactive TTY sessions are unaffected — they still get the confirmation prompt.