chore: untrack target/ build artifacts; expand .gitignore

Build artifacts under target/ were tracked at some point in the
project's history, which has been polluting every commit with
1000+ lines of binary churn. Standard Rust-project hygiene:

  - target/ never belongs in git
  - .rs.bak / .rs.backup local rescue copies don't either
  - per-architecture target/{aarch64,x86_64}-* trees
  - omnimcode-ffi/.cargo/ tooling cache
  - omnimcode-core/test_*.omc local scratch files
  - examples/test_*.omc, examples/enemy_ai.omc one-offs

Untracking via `git rm -r --cached target/` keeps the files on
local disk (cargo build still works); it just stops tracking
them in git. Each future commit will now have a clean diff that
shows actual code/doc changes instead of fingerprint files and
freshly-rebuilt binaries.

Result: 1149 files removed from tracking, ~10K lines of build
artifact noise out of the repo. .gitignore picks up the slack so
they won't sneak back in.

Three directories left for the user to triage separately:
  - examples/agent-decision-evolution/ (related to the GA arm)
  - examples/game-ai-demo/Assets/Editor/ (Unity asset content)
  - omnimcode-gdextension/ (Godot binding stub)

These look like real new content rather than build artifacts;
should probably be added or .gitignored intentionally rather
than swept up in this cleanup pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: RandomCoder-lab

.gitignore

@@ -13,3 +13,20 @@ examples/game-ai-demo/MemoryCaptures/
 
 # OMC consolidation safety net (orphaned src/ tree, kept locally only)
 src.deleted.bak/
+
+# Rust build artifacts — should never be tracked
+target/
+**/*.rs.bak
+**/*.rs.backup
+**/*.rs.backup2
+**/Cargo.lock.backup
+**/Cargo.toml.backup
+
+# Local-only scratch / test files
+omnimcode-core/test_*.omc
+omnimcode-core/qwen_omc_enhanced*.omc
+examples/test_*.omc
+examples/enemy_ai.omc
+
+# Tooling caches
+omnimcode-ffi/.cargo/