fix(agents): canonicalize community routing in AGENTS.md

[tiendungdev]

Summary

• make the root AGENTS.md the canonical home for generated community routing
• keep CLAUDE.md Claude-specific by importing @AGENTS.md exactly once
• migrate the old Gortex-managed CLAUDE.md community block without touching user-authored content
• preserve --analyze overview content in CLAUDE.md and keep project init idempotent
• update adapter docs and the Claude render golden

Verification

• go test -race ./internal/agents/claudecode -run 'ProjectCommunityRouting|ProjectAnalyzeOverview|ProjectModeCreatesCanonicalArtifacts|ClaudeCodeDryRunWritesNothing' -count=1
• go test ./internal/skills -count=1
• go vet ./internal/agents/claudecode ./internal/skills
• go build ./cmd/gortex
• go test ./cmd/gortex -run '^TestAgentsRenderGolden$' -count=1 (clean-LF worktree)
• git diff --check

Windows-only baseline

The broad adapter suite still has two unrelated Windows filesystem/PATH failures: TestResolveHookCommand/foundOnPath and TestEmitPluginBundle_HookHandlerExecBit.

Fixes #543