[Bug] New story creates separate terminal session — should reuse the Untitled session

[realproject7]

Problem

When a user clicks "+ New Story" and starts chatting with Claude, the AI creates a new story folder (e.g., `paper-chair`). But instead of the Untitled session becoming the `paper-chair` session, a new separate terminal session is spawned for `paper-chair`. The original Untitled session is left orphaned with a separate Claude instance.

This means:

• Two Claude sessions running (wasted resources)
• The context from the Untitled session (where the user discussed the story idea) is lost in the new session
• The new `paper-chair` session starts fresh without the brainstorming context

Expected Flow

1. + New Story → launches Untitled terminal session (Claude CLI in `~/.plotlink-ows/stories/`)
2. User chats with Claude, brainstorms ideas
3. Claude creates the story folder (e.g., `mkdir stories/paper-chair` + creates `structure.md`)
4. The Untitled session seamlessly becomes the `paper-chair` session — same terminal, same Claude instance, same conversation context
5. Sidebar updates: "Untitled" → "Paper Chair" (from structure.md title)
6. Terminal tab updates: "Untitled" → "paper-chair"

Current Behavior

• Step 3 happens: Claude creates the folder
• The polling detects the new folder → creates a NEW terminal session for `paper-chair`
• The Untitled session remains as a separate orphaned session
• User now has two tabs: "Untitled" (old) + "paper-chair" (new, no context)

Fix

When the polling in `StoriesPage.tsx` detects a new story folder:

1. Don't spawn a new terminal session for it
2. Instead, rename the oldest Untitled session to the new story name
3. Update the PTY session key in the `ptySessions` Map (backend) from `_new_XXX` to `paper-chair`
4. Update the terminal tab display name
5. The Claude CLI process continues running — same PID, same conversation

Backend change needed

Add an endpoint to rename a session key:
`POST /api/terminal/rename` — body: `{ oldName: "_new_1234", newName: "paper-chair" }`

This moves the PTY entry in the sessions Map without killing/restarting the process.

Frontend change needed

In the polling effect (`StoriesPage.tsx`), when a new story is detected:

• Call the rename endpoint instead of letting a new session auto-create
• Update the session key in the local Map
• Remove the Untitled entry from `untitledSessions` state

Files

• `app/web/components/StoriesPage.tsx` — polling logic for new story detection
• `app/web/components/TerminalPanel.tsx` — session key management
• `app/routes/terminal.ts` — add rename endpoint, update session Map

Acceptance Criteria

• New story creation reuses the Untitled terminal session
• Claude conversation context preserved (no restart)
• Sidebar updates from "Untitled" to story title
• Terminal tab updates from "Untitled" to story name
• No orphaned Untitled sessions after story creation
• Only one Claude process running per story