Plan mode: allow descriptive filenames instead of random generated names

Feature Request

Plan files created during plan mode get auto-generated random names like lovely-chasing-pancake.md, which provide no context about what the plan is for.

Current behavior:
When entering plan mode, the CLI generates a random filename:

Plan file referenced (~/.claude/plans/lovely-chasing-pancake.md)

Desired behavior:
Allow the model (or user) to provide a descriptive name for the plan file, e.g.:

Plan file referenced (~/.claude/plans/add-validation-rules.md)

Why it matters:

• When reviewing ~/.claude/plans/, random names give no indication of what each plan was for
• Users who reference plans in other docs (e.g., CONTEXT.md) need meaningful names
• The model already knows what it's planning — it could derive a short slug from the task description

Possible approaches:

1. Let the model name the plan file based on the task (e.g., kebab-case slug of the plan's purpose)
2. Add a CLI flag like --plan-name to override the random name
3. Prompt the user for a name when entering plan mode

Any of these would be a significant UX improvement over random names.

Environment: macOS, Claude Code 2.x