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

packages/rules-engine/src/index.ts

@@ -6,6 +6,19 @@ export { shouldApplyRule, createContentHash, isDuplicateByContentHash, isDuplica
 export { findProjectRoot, clearProjectRootCache } from "./project-root";
 export { calculateDistance } from "./distance";
 export { findRuleFilesRecursive, safeRealpathSync } from "./scanner";
+export {
+  AGENTS_FILENAME,
+  EXCLUDED_DIRS,
+  GITHUB_INSTRUCTIONS_PATTERN,
+  GLOBAL_DISTANCE,
+  OPENCODE_USER_RULE_DIRS,
+  PROJECT_MARKERS,
+  PROJECT_RULE_FILES,
+  PROJECT_RULE_SUBDIRS,
+  RULE_EXTENSIONS,
+  SOURCE_PRIORITY,
+  USER_RULE_DIR,
+} from "./constants";
 export type {
   AgentsMdCache,
   DirectoryScanEntry,