feat(workflow): block publish on fixed-mode template + config-lockable arg mode

[yogasw]

What

Two related arg_modes improvements for the workflow editor.

1. Fixed-mode + {{...}} blocks publish

A field pinned to mode=fixed whose value contains a Go template silently never renders — it ships a broken URL / body / prompt to production. parse.Validate now emits this as an Error (was a Warning), so it gates publish across every path that promotes a draft:

• UI Publish button (auto-disabled via validationErrorCount)
• MCP workflow_publish (service.Publish → parse.Validate)
• internal publish helpers

Draft SaveDraft does not run Validate, so editing is never blocked. The editor also auto-switches a fixed field to expression the moment you type {{, so the common case never trips the gate.

2. New wick:"mode=fixed|expression" config tag

Schema-driven connector / channel op args (e.g. push_id, title, body) now render the Fixed ⇄ Expression toggle on every field kind (default fixed). A mode= tag greys the toggle out and locks it to that mode; the chosen mode still persists to arg_modes.

Tag	UI
(none)	toggle enabled, defaults to fixed
mode=fixed	toggle greyed, locked fixed
mode=expression	toggle greyed, locked expression

How

• entity.Config.Mode reflected from the mode= tag (normalizeMode accepts only fixed/expression), exposed in args_schema.
• SchemaForm / Field / ArgField honor mode + lockedMode; toggle now renders for all kinds (checkbox swaps to a text input in expression mode).
• parse.validateArgModes returns Errors instead of Warnings.

Behavior

Action	fixed + {{}}
Edit / SaveDraft	allowed; typing {{ auto-flips to expression
Publish (UI)	disabled — red node pin + validation chip
MCP workflow_publish	blocked — validation error

Tests

• Go: pkg/entity (mode tag), parse (argmodes block/allow), connectors, tools/agents — all green.
• FE: npm run check (tsc + vite build) passes.

Notes

Pre-existing, out of scope: engine treats an absent arg_modes key as expression while the UI defaults to fixed; arg_modes is only written on toggle click. Same as every existing node field (url/body/prompt) — not a regression here. Can auto-seed arg_modes[key]="fixed" on first value change as a follow-up if desired.