(remote): show a live remote session before its first transcript exists

[devsuitup]

Closes #278. Measured 2026-09-12 on the host: a CLI launched in tmux writes its descriptor at once and its transcript only after the first prompt; the row was missing for 3 minutes and a manual refresh did not help. Since #272 the row is the only place to stop a remote process.

What

• remote-transport.js listFiles(): each live descriptor (ALIVE filter unchanged) is tagged descriptorOnly by cross-checking the transcript ids from the same ssh call — no extra round trip.
• remote-index.js getPlaceholderSessions(alias): a session-shaped placeholder for each descriptor-only descriptor with a cwd — sessionId = the descriptor's id, folder = encodeProjectPath(cwd), summary = cwd basename, status/statusUpdatedAt, remoteAlias, remoteDescriptorSeen: true, placeholder: true; computed on demand, nothing persisted or mirrored. findSessionAlias(sessionId) for sessions without a cache row.
• main.js: mergePlaceholderSessions(projects) before annotateRemoteAttachable(), so the placeholder gets the same status / attachability treatment; open-terminal's remote branch resolves the alias from the index when getCachedFolder has no row (it used to fall through to a local spawn).
• sidebar.js: no "View messages" control on a placeholder row; stop and attach work as on any live remote row.
• Replacement: once the transcript is scanned the descriptor stops being descriptor-only and the real row, same sessionId, is what the sidebar renders — no swap code. The pid:<n> fallback id is unreachable with today's descriptor validation; stated in the doc.
• Docs: session-cache.md (descriptor-only sessions), session-state.md (placeholder rows).

Proof

• Transport: live descriptor-only kept and tagged, dead dropped. Index: placeholder synthesized; replaced in place when the transcript appears (one session, same id). DOM: placeholder row has .is-alive, the stop control, status text, no transcript button.
• Mutations: dropping the buildPlaceholderSession push → synthesis and replacement tests red; removing the !session.placeholder guard → DOM test red. Both reverted.
• task check: 1318 + 119 pass, 0 fail, pre-existing skips.

Not covered by a direct test: open-terminal's alias fallback branch (handler not source-extracted in tests; findSessionAlias unit-tested). Not verified on the wire.