feat(session connect): Claude Code-style boxed composer + spend/status footer

[hbrooks]

Reshapes the `agent session connect` / `session start --connect` UI to look and behave like Claude Code, and unifies the startup flow so everything renders in one place.

What you see now

Boxed composer — the `›` prompt is framed by a full-width rule above and below (top/bottom borders only, spanning the terminal).

Persistent footer status line (dim, Claude-Code-style):
```
session_1Bacr9… · running · $0.44 total (Last step: $0.03)
```

• Session id is a clickable OSC 8 hyperlink to `app.ellipsis.dev/{login}/sessions/{id}` (degrades to plain text off a TTY / when piped).
• Total = cumulative `total_cost_usd` from the latest Claude Code result.
• Last step = delta between the two most recent results — the cost of the last turn. (This is the `$0.44` running-total vs `$0.03` marginal-turn distinction.)
• Current status shown live.

Lifecycle system lines — on each status transition a dim `●` line lands in the transcript: `creating sandbox` → `sandbox ready · spawning agent process` → `retrying…` / `session complete`.

Flow unification

`start --connect` no longer runs a separate pre-connect braille spinner and then hands off — it drops straight into the connect UI, which now renders the whole sandbox-creation lifecycle itself. A backstop status poll keeps the phase fresh even before the socket attaches, and a terminal status reached before the sandbox ran (a preflight/budget gate) is reported and exits cleanly.

New pure, tested helpers

• `events.ts`: `resultCostUsd`, `foldCosts` (total + last-step), `statusSystemLine`
• `urls.ts`: `hyperlink` (TTY-gated OSC 8)

Note

Footer spend is Claude Code's `total_cost_usd` (Anthropic list price — the number shown in the turn footers today), not the billed session cost (`cost_tokens + sandbox + fee`). It's already in the stream, so no extra polling. Easy to switch to billed cost if preferred.

Testing

• `tsc --noEmit` clean; `vitest run` — 224 passed (10 new).
• Manual: `bun run dev session connect ` (boxed footer + spend) and `bun run dev session start '…' --connect` (lifecycle lines).