Token / cost visibility in /agents

[TroyHernandez]

Summary

`/agents` (and the underlying `subagent_list()` / `format_subagent_list()`) now show model, age, live context size, cumulative input/output tokens, and cumulative cost per subagent. Dense single-line output, one line per agent.

Before:
```
[1] demo task (15.0 min remaining) idle stub-12345678
```

After:
```
[1] demo task (moonshot-v1-8k · 2s · ctx 617/128.0K · 741 in / 5 out · ?) (30.0 min remaining) idle 6777ea48
```

What changed

• `subagent_turn_prompt()` now returns `list(reply, usage)` instead of just the reply string. Two internal callers updated (sync `subagent_query()`, async `subagent_collect()`); saber::blast_radius confirmed nothing external.
• Each registry entry gains cumulative usage counters and a `query_count`. Both wait paths accumulate via the new `subagent_accumulate_usage()` helper. Cost is NA when the provider doesn't surface it (moonshot/ollama); distinguished from $0.
• `subagent_list()` returns the new fields plus a best-effort live token count via `info$session$run()`. Busy agents (with a pending async call) get `NA` — callr can't stack queries on a session with an outstanding call.
• `format_subagent_list()` renders the dense one-line layout. Cost shows `?` when NA, ctx shows `ctx ?` when live tokens are unavailable.
• New helpers in `R/context-budget.R`: `format_age()` and `format_live_ctx()`.

Test plan

• 39 new offline tests in `test_agents_visibility.R` cover the format helpers, `subagent_accumulate_usage()` (NULL-safe, partial usage, running cost), and the full formatter across idle / busy / with-cost shapes.
• `tinytest::test_package("corteza")` — 1555/1555 OK.
• End-to-end against moonshot: spawn → one sync + one async query → cumulative tokens grow correctly, live ctx updates, age advances.

Open follow-ups (not in this PR)

• Cost computation for providers that don't return it natively (would need a per-model pricing table; out of scope here).
• Per-tool-call latency breakdown if that proves useful later.