CLI: Add --output-field flag to extract a single value for piping

[titouanmathis]

Context

Agents frequently chain CLI commands together, passing the output of one as input to another. Currently, extracting a single field requires --format json plus jq:

productive tasks add --title "Bug fix" --project 123 --task-list 456 --format json | jq -r '.id'

Not all environments have jq, and this adds friction for agent workflows.

Proposal

Add --output-field <field> (alias --of) that prints only the value of a specific field from the result:

# Get just the ID of a newly created task
TASK_ID=$(productive tasks add --title "Bug fix" --project 123 --task-list 456 --output-field id)

# Chain: create time entry for a task's service
productive time add \
  --service $(productive tasks get 123 --output-field service_id) \
  --time 60

# Get the URL of a project
productive projects get 123 --output-field productive_url

Behavior

• Prints the raw value (no quotes, no newline decoration) to stdout
• Works with any format's underlying data (extracts from the formatted resource object)
• Supports dot notation for nested fields: --output-field relationships.project.id
• If the field doesn't exist, exit with error code and message to stderr
• Implies --quiet (no spinners, no status messages)

Implementation

• Parse --output-field in the global options
• After the command produces its result data, extract the field and print it
• Could be implemented as a post-processing step in OutputFormatter or in runCommand