[bug] Plan mode does not auto-switch to builder when prompt requires execution

[shreyastelkar]

Problem

When in plan mode and the user sends an actionable prompt (e.g., "create a file", "run dbt build", "file these GitHub issues"), the agent stays in plan mode and refuses to execute. The user must manually switch modes mid-conversation.

Expected behavior

The agent should detect execution intent in the prompt and either:

1. Auto-switch to builder mode, or
2. Prompt the user: "This requires builder mode. Switch now?"

Current behavior

The agent remains in plan mode, responds with read-only analysis, and tells the user it cannot perform the action. The user must know to manually switch modes — which is not obvious, especially for new users.

Steps to reproduce

1. Launch altimate --agent plan
2. Send a prompt like: "Create a staging model for raw_orders"
3. Observe: agent stays in plan mode, refuses to create the file

Notes

• Plan mode is documented as "Minimal (read files only, no SQL or bash)" — the restriction is by design, but the UX gap is the lack of auto-switching or prompting.
• The agent modes docs describe three fixed modes with no switching logic.