fix(compress): Bug 39 — protected tool messages excluded from effectiveMessageIds

[ranxianglei]

Problem

Bug 39 (opencode-acp parity): protected tool messages (e.g. skill, compress) should be hard-excluded from compression. The README documented this as a "Known limitation" — but it was a real bug, not a deliberate simplification.

filterProtectedToolMessages (compress.ts:686) correctly drops protected tool calls and their paired results from directMessageIds. But the result was never written back to effectiveMessageIds. The new block therefore recorded protected messages as covered:

directMessageIds: filteredIds,                    // ← filtered ✓
effectiveMessageIds: [...effectiveMessageIds],    // ← UNFILTERED ✗ (bug)

Consequences:

• Protected tool messages hidden from view (marked as covered by the block)
• If later GC-truncated, the detail is unrecoverable
• Decompress/search incorrectly treat them as compressed

Why it was invisible

Existing tests (protected-content.test.ts) only asserted on directMessageIds — never effectiveMessageIds. So the bug passed all 211 tests.

Fix

compress.ts: after filterProtectedToolMessages, delete removed ids from the effectiveMessageIds Set so both arrays agree:

if (filteredIds.length < directMessageIds.length) {
  const kept = new Set(filteredIds);
  for (const id of directMessageIds) {
    if (!kept.has(id)) effectiveMessageIds.delete(id);
  }
}

Tests

Added effectiveMessageIds assertions to the two skill-protection tests. Verified they fail without the fix (2 failures) and pass with it.

README

Replaced the "Known limitation" note — protected tool messages are now hard-excluded, matching opencode-acp Bug 39.

Test plan

• 211 tests pass (was 211; 2 existing tests strengthened)
• typecheck clean
• New assertions fail without the fix, pass with it