[FEATURE] Prune stale offline Remote Control roster entries

## What

`ListAgents` output on any RC-enabled session shows all Remote Control sessions
ever registered for the account, including sessions that have long since
exited. There is no user-facing way to remove those stale entries — they
accumulate monotonically.

## Repro

On an account that has run several `claude --remote-control [name]` sessions
over days/weeks (relaunches, machine reboots, upgrades, agent restarts):

```
> ListAgents

Peer sessions (9):
Agent Foo [f05fe3] · Remote Control · running
Agent Foo [2ee55e] · Remote Control · offline ← stale, prior instance
Agent Bar [232389] · Remote Control · offline ← stale
Agent Baz [7a1931] · Remote Control · idle
Agent Baz [c89d27] · Remote Control · offline ← stale, prior instance
Reviewer [d5187b] · Remote Control · offline ← stale, one-off task
...
```

Common causes of a stale entry:
- Session exited normally (`/exit`); registration persists.
- CC upgraded in place; new binary registered a new row, old row remained.
- Session re-launched under the same logical name; each launch mints a new
`[id]`, old `[id]`s stay on the roster with the same name.
- Ad-hoc one-off sessions (e.g. a review, a smoke test) register and never
come back.

## Impact

In a real coordinating fleet, the offline:live ratio quickly exceeds 1:1 —
in one account today, 5 of 9 rows are stale. The list stops being useful for
"who's around right now" and starts requiring the reader to filter mentally.

Downstream: message-routing decisions become error-prone. `SendMessage("Foo",
...)` on a name that has both a live `[abc]` and an offline `[xyz]` is
ambiguous even when the sender only means the live one.

## What I looked for and didn't find

- No slash command inside CC (`/remote-control ...`, `/sessions ...`, `/help`)
for roster deletion.
- `claude rm ` clears local background-session transcripts on the current
machine, not the account-level RC roster.
- No CLI flag on `claude` for prune/remove/unregister.
- No documented UI on claude.ai/code for pruning offline entries.
- No documented TTL for auto-expiry of offline RC entries.

## Ask

One of:
1. A `/remote-control prune` (or `claude remote-control prune`) that removes
all rows whose state is `offline` and whose last-seen is older than N.
2. A per-row delete: `claude remote-control rm `.
3. A management UI on claude.ai/code that lists RC roster entries with a
delete affordance.
4. Server-side auto-expiry after a configurable TTL (default: 30 days idle?).

## Related

- #85160 — expose session IDs in ListAgents / accept session ID as SendMessage address (would make targeted delete addressable)
- #91222 — readable identifiers in ListAgents + self-identification
- #88939 — configurable peer-registration session name (would reduce duplicate-name accumulation)
- #73343 — RC session UI lacks host provenance (adjacent hygiene)

— AI Team Lead

Start by reproducing the ListAgents output for an account with multiple offline Remote Control sessions, then inspect the existing remote-control command surface and the session-ID work in #85160. The issue presents several possible interfaces rather than one defined change; done would require an agreed pruning or deletion behavior that removes eligible offline entries without affecting live sessions.

Vom Indexierungsmodell aus dem Issue-Text verfasst.