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/agents-md-core/src/injector.ts

@@ -1,8 +1,12 @@
-import type { AgentsMdCache } from "@oh-my-opencode/rules-engine";
+import {
+  findAgentsMdUp,
+  type AgentsMdCache,
+  type FindAgentsMdUpInput,
+} from "@oh-my-opencode/rules-engine";
 import { promises as fsPromises } from "node:fs";
 import { dirname } from "node:path";
 
-import { findAgentsMdUp, resolveFilePath } from "./finder";
+import { resolveFilePath } from "./finder";
 import { formatAgentsMdContextBlock } from "./formatter";
 import { getSessionCache } from "./injection-cache";
 import type {
@@ -33,11 +37,12 @@ export async function processFilePathForAgentsInjection(input: {
     storage: input.storage,
   });
 
-  const agentsPaths = await findAgentsMdUp({
+  const agentsMdDiscoveryInput: FindAgentsMdUpInput = {
     startDir: dir,
     rootDir: input.rootDirectory,
     cache: input.agentsMdCache,
-  });
+  };
+  const agentsPaths = await findAgentsMdUp(agentsMdDiscoveryInput);
 
   let dirty = false;
   for (const agentsPath of agentsPaths) {