Feature: command to bulk-delete closed sessions

[whtoo]

Problem

Currently, acpx kimi sessions close <name> only marks a session as `[closed]` in the sessions list but does not delete the session files from `~/.acpx/sessions/`. Over time this leads to:

• Hundreds of orphaned session files accumulating on disk (in our case: 190 closed sessions = ~2.4GB)
• The sessions list shows a cluttered mix of open and closed entries
• No way to bulk-clean closed sessions without manual file deletion

Proposed Solution

Add a `prune` subcommand to `acpx sessions`:

```bash

Prune all closed sessions for current agent/cwd

acpx kimi sessions prune

Dry-run: show what would be deleted

acpx kimi sessions prune --dry-run

Prune sessions closed before a given date

acpx kimi sessions prune --before 2026-03-01

Prune sessions older than N days

acpx kimi sessions prune --older-than 7

Also delete session history files (.stream.ndjson)

acpx kimi sessions prune --include-history
```

Behavior

1. Query the sessions list for the current agent
2. Filter sessions with status = `[closed]`
3. Apply optional date/age filters
4. Delete:
   • `{session_id}.json` (session metadata)
   • `{session_id}.stream.ndjson` and `{session_id}.stream.N.ndjson` (if `--include-history`)
5. Print summary: X sessions pruned, Y MB freed

Motivation

• Disk hygiene: sessions can grow large (hundreds of MB per session with long conversations)
• UX: keeps `sessions list` output clean and actionable
• Alternative: implementing file deletion inside `sessions close` is also acceptable, but a separate `prune` command gives users more control

Alternatives Considered

Approach	Pros	Cons
`sessions close` also deletes files	Simple, no new command	Breaking change if users rely on file persistence
`sessions prune` (proposed)	Granular control, dry-run possible	Slightly more complex
Automatic pruning on close	Zero maintenance	May delete data users want to keep

This is purely a CLI UX improvement; the session state is already tracked in memory so no protocol changes needed.