docs(sessions): correct what a failed rebuild is actually likely to be

Ran the feature against a real server for the first time, on an isolated
instance, and two claims in the code turned out to be wrong.

A rebuild that fails after the session is registered was documented as
commonly caused by a CLI binary missing from a freshly booted machine's
PATH. It is not: the resolver finds its binary by absolute path, so PATH
never enters into it, and a server started without claude on PATH restored
every session normally. Nor does an un-enterable workspace fail — tmux falls
back to another directory and the pane comes up there. Neither obvious cause
throws, so the discard path is defended rather than expected, and the
comments now say that instead of naming a cause that cannot happen.

The four review rounds that shaped this path all reasoned about a trigger
none of them could test. The path itself is still worth having, since a mux
failure would reach it, but its comments should not claim a likelihood the
machine disagrees with.

Refs #411

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

src/web/routes/reboot-restore-routes.ts

@@ -220,8 +220,14 @@ export function registerRebootRestoreRoutes(app: FastifyInstance, ctx: RebootRes
           // One entry that will not start must not stop the rest of the pass, and
           // must not leave a registered session with no pane behind it: by this
           // point the session is in `ctx.sessions`, holds a tab-layout slot and has
-          // listeners, and the commonest cause is a CLI binary that is not on the
-          // PATH of a freshly booted machine.
+          // listeners.
+          //
+          // Reaching this is rarer than it looks, measured against a real server:
+          // the CLI resolver finds its binary by absolute path rather than through
+          // PATH, and tmux falls back to another directory rather than failing when
+          // it cannot enter the workspace, so neither of the two obvious "freshly
+          // booted machine" failures throws. What is left is the mux layer itself
+          // failing, which is why this path is defended rather than expected.
           console.error(`[reboot-restore] failed to rebuild ${entry.sessionId}:`, err);
           // Not cleanupSession(): that is the user-initiated delete, and it would
           // count this session's historical tokens into the lifetime totals, demote