feat(agent-dialog): require default folder for provider CLI agents

[pikann]

Summary

Provider CLI agents created without a default folder never start their conversation when assigned a task. The create-agent dialog now requires a default folder for provider CLI agents:

• The Default folder field is marked required (*) and the "No default folder" option is hidden, so a folder must be picked or created.
• The Create button stays disabled until both the default environment and the default folder are set.
• DefaultFolderSelect gains an optional required prop (default false), so the agent detail page behaves as before.

Not included: server-side enforcement. A direct API call can still create a provider CLI agent without default_folder_id, and existing agents without a folder are unaffected.

Type of Change

• Other

Checklist

• The change is focused and scoped.
• Related documentation is updated. (No docs affected.)
• New structure or direction is explained clearly.
• I avoided unnecessary detail or premature abstraction.

🤖 Generated with Claude Code

[pullfrog]

ℹ️ No critical issues — two minor suggestions.

Reviewed changes

• create-agent-dialog.tsx — provider_cli step2Valid now also requires defaultFolderId, and the Default folder field is marked * and passes required to the select.
• environment-folder-select.tsx — DefaultFolderSelect gains an optional required prop (default false) that hides the "No default folder" item and skips the NO_FOLDER → "" mapping; mirrors the existing DefaultEnvironmentSelect.required.

ℹ️ Required-folder gate is UI-only

The dialog now blocks the broken case, but the server still accepts a provider_cli agent created without default_folder_id through any non-dialog client (direct API, MCP, older frontends), so the "conversation never starts" failure remains reachable outside this form. The PR body explicitly scopes server enforcement out — worth confirming whether a follow-up is planned, since the failure mode is what motivated the change.

Technical details

# Server-side enforcement for provider_cli default folder

## Affected sites
- `apps/web/src/components/projects/agents/create-agent-dialog.tsx:516` — UI gate added here
- server agent-create path — no equivalent requirement for `agentType == provider_cli` + missing `default_folder_id`

## Required outcome
- Decide whether provider_cli agents must reject creation without a default folder server-side (the dialog already surfaces `ApiErrorCode.AgentDefaultFolderInvalid`).

## Open questions for the human
- Is a server-side validation follow-up planned, or is dialog-only enforcement the intended long-term boundary?

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[pullfrog]

When required and no folder is picked yet, value is "", which matches no item — and unlike DefaultEnvironmentSelect (which passes a placeholder), this <SelectValue /> has none, so the trigger renders blank. This is reachable for a freshly created environment whose folders is null/empty. environments.picker.folderPlaceholder already exists in every locale.

Technical details

# Missing placeholder on required folder select

## Affected sites
- `apps/web/src/components/projects/environments/environment-folder-select.tsx:168` — `value={required ? value : value || NO_FOLDER}`
- `apps/web/src/components/projects/environments/environment-folder-select.tsx:198` — `<SelectValue />` lacks a `placeholder`
- `apps/web/src/components/projects/environments/environment-folder-select.tsx:96-102` — `DefaultEnvironmentSelect` sets one for its required case

## Required outcome
- The required folder trigger shows a prompt before a folder is chosen, consistent with the required environment select.

## Suggested approach (optional)
- `<SelectValue placeholder={required ? t("environments.picker.folderPlaceholder") : undefined} />`