fix: CLI context pollution between sessions

[continue]

Summary

Fixes context pollution issue in the Continue CLI where sessions would retain state from previous invocations, causing the LLM to reference conversations from prior sessions even when starting fresh (without --resume).

Problem

As reported in #7916 and described in CON-4530, the CLI was maintaining state in singleton services across separate CLI invocations. This meant:

1. Running cn "Tell me about dogs", having a conversation, then exiting
2. Running cn "What were we discussing?" in a new session
3. The CLI would incorrectly reference the previous conversation about dogs

Root Cause

Three main sources of state pollution:

1. SessionManager singleton - maintained currentSession across invocations
2. ChatHistoryService - kept undo/redo stacks and memoized state
3. ServiceContainer - global singleton persisted service instances

Solution

1. Session Management (session.ts)

• Added SessionManager.reset() static method for proper cleanup
• Ensures singleton can be reset for testing and clean state

2. Chat Initialization (chat.ts)

• Updated initializeChatHistory() to explicitly create a new session when neither --resume nor --fork flags are used
• This ensures every fresh CLI invocation starts with a clean session

3. Service Cleanup (ChatHistoryService.ts)

• Clear undo/redo stacks during initialization
• Reset memoized snapshot cache
• Prevents state leakage between service reinitializations

4. Tests (session.test.ts)

• Added comprehensive "session isolation" test suite
• Verifies new sessions don't inherit history from previous sessions
• Tests concurrent session operations
• Validates proper state clearing during transitions

Testing

The fix includes three new test cases:

• should not pollute new sessions with previous session history
• should create independent sessions for concurrent operations
• should properly clear session state when transitioning between sessions

Behavior Changes

Before

# Session 1
$ cn "Tell me about dogs"
> Dogs are loyal companions...
$ # Exit

# Session 2 (WRONG - references previous conversation)
$ cn "What were we discussing?"
> We were discussing dogs...

After

# Session 1
$ cn "Tell me about dogs"
> Dogs are loyal companions...
$ # Exit

# Session 2 (CORRECT - clean session)
$ cn "What were we discussing?"
> I don't have context about a previous discussion...

Flags Still Work as Expected

• cn --resume: Continues most recent session ✅
• cn --fork <id>: Forks from specific session ✅
• cn (no flags): Clean new session ✅

Related Issues

Fixes CON-4530
Closes #7916

---

This agent session was co-authored by Brian Douglas and Continue.

---

Summary by cubic

Fixes CLI context pollution so new sessions start clean and don't inherit history from previous runs. Applies to fresh invocations; sessions only carry over when using --resume or --fork. Addresses Linear CON-4530.

• Bug Fixes
  • Start a new session by default in chat initialization when no resume/fork flags are used.
  • Clear undo/redo stacks and memoized snapshot on ChatHistoryService init to prevent leakage.
  • Add SessionManager.reset for explicit cleanup and add tests that verify session isolation and clean transitions.

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