[feature request] Add session deletion support in TUI

Summary

Currently there is no way to delete or manage sessions from within the kimi-code TUI. The only way to remove a session is to manually delete its directory under ~/.kimi-code/sessions/<workspace>/ and edit session_index.jsonl to remove the corresponding JSON line. This is error-prone and tedious, especially when orphaned index entries accumulate over time.

Proposed change

Add a slash command (or extend an existing one) that allows users to list, select, and delete sessions directly from the interactive TUI.

Desired behavior

• List all sessions for the current workspace (similar to /sessions for resuming, but with management options)
• Allow selecting one or more sessions and deleting them
• Clean up both the session directory on disk and the corresponding entry in session_index.jsonl
• Optionally warn when a session has no matching directory (orphaned index entry)

Example interaction

> /sessions-delete

  session_c391db2e  2026-07-19 15:30  C:\Users\Administrator\Documents\opencode project\test3
  session_b895b8d6  [orphaned - no data]

  ↑/↓ to select, Space to mark, Enter to delete marked, Esc to cancel

Benefits

• Safer than manual file editing (no risk of corrupting session_index.jsonl JSON)
• Prevents orphaned entries from accumulating
• Gives users control over disk space used by old sessions
• Consistent with the existing /sessions / /resume workflow

Workaround (current)

Users currently have to:

1. rm -rf ~/.kimi-code/sessions/<workspace>/<session_id>
2. Manually edit ~/.kimi-code/session_index.jsonl to remove the matching line

This is fragile — a malformed JSON line breaks the session index.

Environment

• kimi-code version: 0.27.0
• Platform: Windows / win32-x64

Start by tracing the existing /sessions and /resume workflow in the TUI, then inspect how ~/.kimi-code/sessions/<workspace>/ and session_index.jsonl are read and updated. Define the deletion flow around selection, confirmation, directory cleanup, and index-entry removal, including orphaned entries. Done means selected sessions are safely removed without corrupting the index.