Swap Enter/Shift+Enter in agents prompt editor

[aletc1]

Summary

Swap the submit/newline keys in the shared AgentsMentionsEditor:

• Enter → insert a newline (default contenteditable behavior)
• Shift+Enter → submit
• Opt+Shift+Enter → force-submit (stop current stream, bypass queue, send immediately)

Applies to both the new-workspace ("What do you want to get done?") input and the active-chat input, since both render the same editor.

Closes #197

Why

Multi-line prompts were easy to accidentally send with a stray Enter. Making Enter a newline and requiring a modifier to submit matches the pattern users already expect from other chat UIs for longer-form composition.

Changes

• src/renderer/features/agents/mentions/agents-mentions-editor.tsx — flipped the shiftKey check in the Enter branch of the editor's onKeyDown handler (the core one-line behavior change). Plain Enter now falls through to the native contenteditable newline; dropdown (@ mention / / slash) handling is unchanged.
• Updated comments in chat-input-area.tsx, active-chat.tsx, and new-chat-form.tsx so any "Opt+Enter" / "press Enter to send" references now say "Opt+Shift+Enter" / "press Shift+Enter to send".

IME composition (isComposing) guard is preserved — composing in CJK IMEs still commits on Enter without submitting.

Test plan

• New workspace: select a project, type text, press Enter → newline; press Shift+Enter → submits multi-line prompt
• Active chat: same Enter/Shift+Enter behavior
• Mentions / slash: typing @ then Enter still selects mention; typing / then Enter still selects command
• Force-submit: during an in-flight stream, Opt+Shift+Enter stops the stream and sends immediately
• IME: commit a CJK composition with Enter — composition commits, no submit

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