Show and attach to every tmux session on an SSH host (opt-in, read-only by default, attached status) #159

User story

As a user I would like to see and attach to every tmux session on an SSH host from eve, not only the ones relay created, so I can watch and steer long-running work there (for example parallel Claude sessions started by a coordinator) from the browser or my phone, without an ssh terminal.

Background

Today GET /api/projects/{id}/persistent-sessions runs tmux ls on the host, then keeps only names that parse as relay-<project8>-<template>-<n> for that project. Reattach (POST /api/terminals with persist_session) refuses any other name. Other sessions on the host are invisible.

Security: a host project already grants a login shell on that host, from which tmux attach -t <anything> works. This feature adds convenience, not authority.

Acceptance criteria

• A host setting "Show all tmux sessions" exists, and its default is off.
• With the setting off, session listing and attach behave as they do today.
• With the setting on, Remote Sessions shows a second group, "Other sessions on ".
• The "Other sessions" group lists every session on the host that is not relay-named.
• Each row shows the session name, its creation time and its attached status.
• Each session in both groups shows how many clients are attached besides this relay, such as "watched by 1 other".
• The count is session_attached minus relay's own live terminals on that session.
• Hovering or expanding a session lists each client's terminal and whether it is read-only (tmux list-clients -F).
• Opening an "other" session attaches with tmux attach -f read-only,ignore-size -t <name>.
• A read-only attach accepts no input.
• Eve's client never resizes the shared window, so a phone-sized client does not reflow the TUI for others.
• An explicit "take control" action re-attaches without read-only.
• Take control keeps ignore-size unless the user also chooses to resize.
• Before attaching, relay confirms the exact session name is present in the host's session list.
• A name that is only a prefix match, such as relay-15 against relay-156, is never attached.
• Relay does not offer kill for a session it did not create, in the API or in the UI.
• Closing the tab, an ssh drop or a relay restart detaches the session and never kills it.
• On a host with tmux older than 3.2, the "other" group is still listed.
• On such a host, attach reports that it is unsupported and does not fall back to a full attach.

What I'd notice

Nothing, until I turn on "Show all tmux sessions" for a host. Then I see its other sessions, read-only by default.

Tasks

• T1 · relay: host setting, and listing of every session with client counts and client details (API and wire shape)
• T2 · relay: attach to a non-relay session, read-only plus ignore-size by default, take-control variant, exact-name guard, no kill
• T3 · eve: the "Other sessions" group in Remote Sessions, attached status on both groups, read-only badge and a take-control action
• T4 · devbox journey: list and read-only attach to a tmux session the journey starts itself, and confirm the window size doesn't change