feat(sessions): offer to rebuild the sessions a host reboot destroyed

A host reboot takes the tmux server down with it, so every pane dies,
reconciliation finds nothing to attach to, and the board comes up empty.
Picking yesterday's work back up meant finding each conversation in history
and resuming it by hand, one at a time.

The boot pass now works out what the reboot killed and leaves it on offer.
It runs inside restoreMuxSessions(), in the window where reconciliation has
reported the dead sessions and cleanupStaleSessions() has not pruned their
records yet, which is the only place the records can still be read. The
board shows a banner, and nothing is created until the user clicks it.

A click rather than an automatic restore is what makes the reboot heuristic
acceptable. The heuristic cannot tell a reboot from a crash that took tmux
down inside the same window, so it decides whether to ASK, never whether to
act: a wrong yes costs a line of text the user dismisses instead of N CLI
processes nobody asked for.

Four things are re-checked when the click arrives rather than trusted from
boot, because hours can pass and the board moves on. The owner's privilege
grant re-resolves through the env clamp. The workspace must still be on
disk. A conversation the user already resumed by hand from the Resume list
is skipped, since two panes running --resume on one conversation would
fight over the same transcript. Entries leave the plan synchronously before
the first await, and the route is single-flighted, so a double-click or two
devices cannot both reach the same entry.

A restored session comes back attached, idle and disarmed. Respawn
controllers and Ralph loops are deliberately not re-armed: a machine that
just came up is the worst moment to turn an autonomous run loose. Its
workspace hooks are installed by the restore route itself, because the
boot-time sweep sits behind a gate that is false after a reboot and has
finished long before the click; without them a session goes silently blind,
with no stop or idle events for respawn, no Approvals Inbox item and no red
tab on a blocking dialog. Stats collection starts the same way.

The pane is new, so the conversation continues and the terminal scrollback
does not. The banner says so rather than letting an empty pane read as a
broken restore.

The plan lives in memory only. A server restart drops it, which costs the
convenience this adds and never the conversation: the conversation is the
transcript under ~/.claude/projects, which the Welcome screen's Resume list
and the Session Manager already read, so a dropped plan returns the user to
resuming by hand.

clampEnvOverridesForOwner moves to src/session-env-clamp.ts, since the
question it answers is about session privilege rather than about HTTP and
it now has a caller outside the route layer. Its test hook stays re-exported
from session-routes.ts.

Claude sessions only for this pass. The other CLIs name their thread in
their own config object, which this does not thread through yet. Remote and
docker sessions are skipped on purpose, because both need another host or a
container to be up and a freshly booted machine cannot promise either.

Refs #411

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.changeset/reboot-restore-banner.md

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+---
+'aicodeman': minor
+---
+
+Offer to rebuild the sessions a host reboot destroyed. A reboot takes the tmux server down with it, so every pane dies and the board comes up empty. Codeman now works out what was running, and the board offers to restore it behind a click. The conversations come back; the terminal scrollback does not, and the banner says so.