feat(shell): add /delete command for session removal (#1783)

[bloodycoder]

Summary

Implements #1783 by adding /delete command (with /remove alias) for session removal in shell slash commands.

Scope / behavior

• New command: /delete [session_id]
• Alias: /remove
• Safety constraints:
  • cannot delete current session
  • invalid session id is rejected early
  • v1 deletion is limited to current work_dir
• Confirmation hook is injectable for tests.
• Stable user-facing messages for success/failure paths.

Implementation details

• Added guarded delete flow in shell slash command registry.
• Added helper functions for:
  • session id validation
  • delete confirmation
  • metadata cleanup handling
• Added docs updates (EN/ZH) for slash command reference + data location notes.

Files changed

• src/kimi_cli/ui/shell/slash.py
• tests/ui_and_conv/test_shell_slash_commands.py
• docs/en/reference/slash-commands.md
• docs/zh/reference/slash-commands.md
• docs/en/configuration/data-locations.md
• docs/zh/configuration/data-locations.md

Test plan

Added/updated tests around:

• command registration and alias parity
• by-id delete path
• picker-based delete path
• failure paths (invalid id / not found / cross-work-dir / current session)

Current status

Draft PR due to local environment mismatch:

• local Python: 3.9
• repo requires: >=3.12 (.python-version is 3.14)
• missing tools: uv, tmux

I will rerun and attach full passing logs once environment is aligned.