Interactive `gremlins --watch`: arrow-key selection with inline expanding log tail

[xbrianh]

Proposal

gremlins --watch becomes a single-pane interactive TUI:

• Fleet rows render as today (KIND/ID/STAGE/LIVENESS/AGE/BOSS/CLIENT/DESCRIPTION).
• ↑/↓ move selection; the selected row is highlighted.
• The selected row expands inline: dimmed live log tail appears directly below it; rows after the selection shift down. Switching selection collapses the previous row and expands the new one.
• q quits. Other keys (drill-in, stop, land) optional follow-ups.

The log tail follows a log -f of the selected gremlin's log file. Bounded buffer (e.g. last 500 lines or so) to keep redraws cheap; auto-scrolls. Dimmed styling matches Claude Code's pattern of streaming output below a prompt.

Why single-pane (accordion) instead of split

• Spatial locality: log lines appear next to the gremlin they belong to.
• Self-adjusting: log gets whatever vertical space the rest of the table doesn't claim. Small fleet, lots of log; large fleet, less log.
• No fixed-height pane waste when the fleet is small.

Implementation notes

• textual is the natural fit — collapsible rows are first-class and async log streaming integrates cleanly.
• Fall back to today's static refresh when stdout isn't a TTY (piped output, etc.).
• Selection persistence: when the table refreshes (a gremlin finishes, a new one launches), keep the cursor on the same gremlin ID, not the same row index.
• Log file source: each gremlin's <state-dir>/log is the canonical stream — same file gremlins log -f reads.

Acceptance

• Launching gremlins --watch in a TTY shows the table with one row pre-selected and its log expanded below.
• ↑/↓ moves selection; previous row collapses, new row expands with its log.
• q exits cleanly.
• Non-TTY invocation falls back to the current static refresh behavior.

Dependencies

Depends on #736 (surface active inner stage for container stages) — the STAGE column already renders the nested path before this TUI lands.