refactor(rules-engine): centralize rule constants and AGENTS.md walk-up

Promote the project-rule constants (PROJECT_MARKERS, PROJECT_RULE_SUBDIRS, PROJECT_RULE_FILES, OPENCODE_USER_RULE_DIRS, USER_RULE_DIR, GITHUB_INSTRUCTIONS_PATTERN, RULE_EXTENSIONS, AGENTS_FILENAME, etc.) and the findAgentsMdUp walk-up helper out of the agents-md-core and rules-injector adapters and into @oh-my-opencode/rules-engine, the single owner of rule discovery.

- packages/agents-md-core/ drops the findAgentsMdUp/AgentsMdDiscoveryInput wrappers (now sourced directly from rules-engine) and its constants module re-exports AGENTS_FILENAME from rules-engine instead of duplicating it.
- src/hooks/directory-agents-injector/finder.ts pulls findAgentsMdUp from rules-engine directly while still re-exporting resolveFilePath from agents-md-core.
- src/hooks/rules-injector/constants.ts becomes a pure re-export shim over the rules-engine constants.

Add packages/agents-md-core/src/injector.test.ts to lock the root-skipping AGENTS.md injection order so future changes to findAgentsMdUp cannot silently regress the [Directory Context: ...] block format the injector emits.

Tests: bun test packages/agents-md-core packages/rules-engine src/hooks/directory-agents-injector src/hooks/rules-injector

src/hooks/directory-agents-injector/finder.ts

@@ -1,4 +1,2 @@
-export {
-  findAgentsMdUp,
-  resolveFilePath,
-} from "@oh-my-opencode/agents-md-core";
+export { resolveFilePath } from "@oh-my-opencode/agents-md-core";
+export { findAgentsMdUp } from "@oh-my-opencode/rules-engine";

src/hooks/rules-injector/constants.ts

@@ -2,30 +2,12 @@ import { join } from "node:path";
 import { OPENCODE_STORAGE } from "../../shared";
 export const RULES_INJECTOR_STORAGE = join(OPENCODE_STORAGE, "rules-injector");
 
-export const PROJECT_MARKERS = [
-  ".git",
-  "pyproject.toml",
-  "package.json",
-  "Cargo.toml",
-  "go.mod",
-  ".venv",
-];
-
-export const PROJECT_RULE_SUBDIRS: [string, string][] = [
-  [".github", "instructions"],
-  [".cursor", "rules"],
-  [".claude", "rules"],
-  [".omo", "rules"],
-];
-
-export const PROJECT_RULE_FILES: string[] = [
-  ".github/copilot-instructions.md",
-];
-
-export const GITHUB_INSTRUCTIONS_PATTERN = /\.instructions\.md$/;
-
-export const USER_RULE_DIR = ".claude/rules";
-
-export const OPENCODE_USER_RULE_DIRS = [".omo/rules", ".opencode/rules"];
-
-export const RULE_EXTENSIONS = [".md", ".mdc"];
+export {
+  GITHUB_INSTRUCTIONS_PATTERN,
+  OPENCODE_USER_RULE_DIRS,
+  PROJECT_MARKERS,
+  PROJECT_RULE_FILES,
+  PROJECT_RULE_SUBDIRS,
+  RULE_EXTENSIONS,
+  USER_RULE_DIR,
+} from "@oh-my-opencode/rules-engine";