Add `dotagent next` — chronological view of upcoming agent fires

Today there is no way to ask dotagent "what is going to run in the next hour / day?" without inspecting each agent's manifest manually. The adaptive scheduler in `dotagent-scheduler` already computes the next fire time for every `(agent, schedule)` — that information just is not exposed at the CLI.

A `dotagent next` subcommand would show a chronological calendar view of upcoming executions across every discovered agent. Useful for: validating you didn't stack everything on the same minute, planning maintenance windows, debugging "why is this not running yet".

## Proposal

A new subcommand:

```
dotagent next # next 24h, all agents
dotagent next --window 7d # next 7 days
dotagent next --agent foo # only one agent
dotagent next --json # machine-readable
```

Output should be a chronologically sorted table:

```
2026-05-20 10:00 hello-fish/manual interval(10080m)
2026-05-20 10:15 databricks-cost-daily/morning cron(weekday=mon-fri 10:15)
2026-05-20 11:00 hello-go/hourly interval(60m)
```

## Acceptance criteria

- [ ] Default window is 24h. Configurable via `--window` (accepts `30m`, `4h`, `7d`).
- [ ] All schedule types render correctly: `interval`, `cron`, `expression` (depends on #5).
- [ ] `--json` emits a stable array of `{agent, schedule, fires_at, schedule_kind}` records.
- [ ] Agents with no firing in the window are listed at the bottom with reason ("next fire is in 14d").
- [ ] No new fields needed in `agent.toml`. Pure read from existing scheduler functions.

## Where to start

- `crates/dotagent-scheduler/src/lib.rs` — `next_after` is already there for `interval` and `cron`.
- `crates/dotagent/src/discovery.rs` — to walk every agent.
- `crates/dotagent/src/commands/output.rs` — the shared renderer convention.

## Non-goals

- Triggering anything. Read-only command.
- Showing past runs (that is `dotagent history`, sibling issue).

Start with crates/dotagent-scheduler/src/lib.rs and its next_after function, then trace agent discovery in crates/dotagent/src/discovery.rs and rendering conventions in crates/dotagent/src/commands/output.rs. Define the command around existing scheduler data, including filters and window parsing; it is done when sorted text and stable JSON output cover all schedule kinds and explain agents with no fire in the window.

Written by the indexing model from the issue text.