[FEATURE] /branch should register forked sessions as navigable entries in claude agents

Problem Statement

When using claude agents as a session manager, there is no way to start two interactive feature branches from a shared context and manage both within the agents interface.

The two existing commands each solve half the problem:

• /branch - creates an interactive fork with full shared context, but the new session never appears in claude agents. It can only be reached via claude -r <session-id> in a separate terminal.
• /fork <directive> - does appear in claude agents, but is a one-way task worker. You cannot interact with it conversationally.

None of the available options give all three properties simultaneously:

| Approach | Shared context | Interactive | Visible in claude agents |
|---|---|---|---|
| /branch twice | Yes | Yes | No |
| /fork <directive> twice | Yes | No | Yes |
| Two terminals with claude -r <id> | Yes | Yes | No (manual) |

Use Case

I have a dev session with accumulated context (environment set up, codebase explored, decisions made). I want to start two features from that exact point and work on each interactively, switching between them in claude agents the same way I switch between any other agents.

Today I have to manage session IDs manually across multiple terminals. The agents panel is blind to both branches.

Proposed Behaviour

When /branch is called inside an active session:

1. The forked session is registered as a child entry in claude agents, linked to the parent
2. Both the parent and child are navigable and interactive from the agents TUI
3. The forkedFrom metadata already written to the session file (it exists today) is used to model the parent-child relationship

Related

• #32631 - broader branching spec (fork, merge, tree navigation)
• #24123 - /fork should open in a new terminal tab
• #55347 - /rewind promises a fork but doesn't create one