feat: enable direct messaging to subagents from TUI

[cetagostini]

Summary

• Add POST /:sessionID/inbox/send server endpoint that wraps Inbox.send() for direct subagent messaging
• Add onCustomSubmit prop to Prompt component allowing custom submit routing (skips model/workspace checks)
• Show input bar when viewing a running, pending, or idle subagent — users can now type directly to subagents without relaying through the main agent

Motivation

When navigating to a subagent's conversation view in the TUI, the input bar was hidden. All communication had to go through the main agent via actor send, adding latency. This PR enables direct messaging by:

1. Exposing the existing Inbox.send() infrastructure as an HTTP endpoint
2. Routing the TUI prompt's submit to that endpoint when viewing a subagent

If a sub-agent goes away from the main goal, you can provide small notch to put it back on track. If main agent miss indications, user can provide without let the agent go crazy.

Changes

Server (packages/opencode/src/server/routes/instance/session.ts)

• New POST /:sessionID/inbox/send endpoint accepting { actorID, content, type? } and returning { inboxID }
• Reuses the same inboxServiceRef.current.send() that the actor send tool uses

TUI Prompt (packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx)

• New onCustomSubmit prop on PromptProps: when provided, the submit function calls it instead of the default session.prompt path
• The custom path skips model selection, workspace checks, and slash command handling — just sends the text and clears the input

TUI Session (packages/opencode/src/cli/cmd/tui/routes/session/index.tsx)

• New subagentInputVisible memo: returns true when viewing a subagent with status running, pending, or unknown (idle-without-outcome)
• Renders a second <Prompt> with onCustomSubmit that POSTs to the inbox send endpoint
• SubagentFooter (navigation + status) remains visible above the input bar

Testing

1. Spawn a persistent peer via session create --topic test
2. Navigate to the child session in the TUI
3. Verify the input bar appears alongside the subagent footer
4. Type a message and submit — verify it arrives in the subagent's inbox and triggers a turn
5. Verify the subagent's response appears in its conversation view
6. Navigate back to main — verify the main agent's input bar works normally
7. Test with ephemeral subagents — verify input is disabled after terminal

Out of Scope

• Backend Inbox changes (already complete)
• Permission system changes (handled by PR fix(permission): background subagents inherit parent grants instead of re-prompting/failing closed #1700)
• SDK regeneration (using direct fetch instead)

[cetagostini]

@wqymi and @MiMoHardFather what do you think?