fix(sequentialthinking): prevent unbounded memory growth in thought history

[Limitless2023]

Summary

Fixes #2912 — The SequentialThinkingServer stores all thoughts in memory arrays (thoughtHistory and branches) that grow without bound across the server process lifetime. In long-running sessions (6-8+ hours of frequent use), this can lead to 10GB+ RAM consumption.

Root Cause

thoughtHistory: ThoughtData[] and branches: Record<string, ThoughtData[]> are append-only — thoughts are pushed but never removed or trimmed. Since the server is a long-lived stdio process, memory accumulates indefinitely.

Changes

1. Configurable max history with auto-trimming (lib.ts)

• Added SEQUENTIAL_THINKING_MAX_HISTORY environment variable (default: 1000)
• When thought count exceeds the limit, oldest thoughts are trimmed via splice()
• This bounds memory usage to a predictable maximum regardless of session length

2. Explicit clearHistory() method (lib.ts)

• New public method that resets both thoughtHistory and branches to empty
• Returns metadata about what was cleared (previous thought/branch counts)

3. New sequentialthinking_clear tool (index.ts)

• Registers a tool that clients can invoke to explicitly free memory
• Useful between distinct thinking sessions within the same server process

4. Documentation (README.md)

• Added environment variables table documenting both DISABLE_THOUGHT_LOGGING and the new SEQUENTIAL_THINKING_MAX_HISTORY

5. Tests (lib.test.ts)

• clearHistory — verifies thoughts and branches are fully reset
• Memory management — verifies history trimming at default limit
• Environment variable — verifies custom SEQUENTIAL_THINKING_MAX_HISTORY is respected

Testing

All 17 tests pass:

✓ __tests__/lib.test.ts (17 tests) 4ms
Test Files  1 passed (1)
     Tests  17 passed (17)

Design Decisions

• Default limit of 1000: Conservative — retains substantial context while preventing runaway growth. A single thought is typically a few hundred bytes of text, so 1000 thoughts ≈ a few hundred KB.
• Trim oldest first: Most recent thoughts are the most relevant for ongoing reasoning. Oldest thoughts are the least likely to be referenced.
• Separate clear tool: Allows explicit memory management without losing the auto-trim safety net.