fix(soul): restore plan-mode tool bindings after /init creates throwaway soul

[nankingjing]

Bug

Fixes #2478 — when /init runs, it creates a throwaway KimiSoul(soul.agent, ...) that shares the live soul's agent (and therefore the same tool instances). The throwaway soul's __init__ calls _bind_plan_mode_tools(), which rebinds the shared ExitPlanMode, EnterPlanMode, WriteFile, and StrReplaceFile instances to closures that reference the throwaway soul's state.

After /init completes and the throwaway soul is discarded, those shared tools are left pointing at stale closures whose _plan_mode is always False. This creates a state split:

• The plan-mode dynamic injection (reading soul.plan_mode on the live soul) correctly shows "Plan mode is active".
• ExitPlanMode (reading the stale closure on the dead soul) reports "Not in plan mode".

The workaround "call EnterPlanMode again, then ExitPlanMode" works because EnterPlanMode toggles the stale throwaway soul's _plan_mode to True.

Fix

1. New public KimiSoul.rebind_plan_mode_tools() method in src/kimi_cli/soul/kimisoul.py — wraps the existing _bind_plan_mode_tools() for external callers.

2. Call soul.rebind_plan_mode_tools() in the /init handler (src/kimi_cli/soul/slash.py) inside a try/finally so bindings are always restored even if the INIT prompt run fails.

3. Two regression tests in tests/core/test_plan_mode.py:

   • test_second_soul_clobbers_binding_and_rebind_restores — demonstrates that constructing a second soul over a shared agent clobbers the ExitPlanMode binding, and rebind_plan_mode_tools() restores it.
   • test_init_slash_command_preserves_plan_mode_binding — end-to-end: runs the actual /init handler and verifies the live soul can enter plan mode afterwards.

Root cause

The plan-mode tools (ExitPlanMode/EnterPlanMode/WriteFile/StrReplaceFile) are mutable instances stored on the shared agent toolset. Their bind() methods store closures over a specific KimiSoul via capturing self in _bind_plan_mode_tools. When two souls share the same agent, whichever soul calls _bind_plan_mode_tools last "wins" — the other soul loses its bindings.

Verification

• Root cause confirmed by code-level trace: all three KimiSoul construction sites examined; only /init shares a live agent. The binding mechanism (closures capturing self._plan_mode) and the over-write-on-rebind behavior confirmed. The workaround mechanism (EnterPlanMode flips the stale soul's flag) also explained.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[nankingjing]

Reviewed — the rebind fix in a finally block correctly restores shared plan-mode tools after /init, and the tests are thorough. Ready for review.