feat: sync rules across all harnesses + agents to opencode/kilo

Add Rule as a first-class canonical kind (rules/<name>.md, loaded like
agents) and propagate it the way each harness actually loads it:

- claude-code: native ~/.claude/rules directory symlink (preserves rule
  path-scoping frontmatter); rules NOT folded into CLAUDE.md.
- opencode/kilo/zed/crush: fold rule bodies into the global AGENTS.md
  always-on instructions file (frontmatter stripped). crush has no global
  instructions file, so register the absolute AGENTS.md path in
  options.context_paths (defaults are re-prepended at load, so nothing is
  clobbered).
- goose: fold into ~/.config/goose/.goosehints.
- cagent: fold into the per-agent instruction.

Also:
- Wire markdown agents to opencode (~/.config/opencode/agents) and fix the
  kilo agents symlink: plural ~/.config/kilo/agents (singular "agent" is a
  silent no-op on current Kilo CLI).
- Add ManagesAgents/ManagesRules to HarnessCapabilities + ImportResult.Rules;
  claude-code/opencode import rules/agents round-trip.
- Centralize instruction selection in Bundle.InstructionText /
  InstructionTextWithRules (replaces per-adapter override duplication).
- init scaffolds + imports rules; README capability matrix gains
  Agents/Rules columns with native-vs-folded footnotes.

Grounded each harness's sink against official docs + config source this
session. Gate: gofmt, go vet, go test -race ./..., golangci-lint (0),
govulncheck (clean).

internal/adapters/claudecode/rules_test.go

@@ -0,0 +1,47 @@
+package claudecode
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/lukaszraczylo/harness-sync/internal/adapter"
+	"github.com/lukaszraczylo/harness-sync/internal/canonical"
+)
+
+func TestClaudeCodeRendersRulesSymlink(t *testing.T) {
+	home := t.TempDir()
+	ad := New(WithHome(home))
+	b := &canonical.Bundle{Root: "/canon"}
+	fs, err := ad.Render(b)
+	require.NoError(t, err)
+
+	seen := map[string]adapter.File{}
+	fs.ForEach(func(f adapter.File) { seen[f.Dest] = f })
+
+	rulesDest := filepath.Join(home, ".claude", "rules")
+	assert.Equal(t, adapter.SymlinkDir, seen[rulesDest].Kind)
+	assert.Equal(t, "/canon/rules", seen[rulesDest].SymlinkTarget)
+
+	// Rules are delivered natively as a directory — never folded into CLAUDE.md.
+	claudemd := string(seen[filepath.Join(home, ".claude", "CLAUDE.md")].Content)
+	assert.NotContains(t, claudemd, "/canon/rules")
+}
+
+func TestClaudeCodeImportsRules(t *testing.T) {
+	home := t.TempDir()
+	rulesDir := filepath.Join(home, ".claude", "rules")
+	require.NoError(t, os.MkdirAll(rulesDir, 0o750))
+	require.NoError(t, os.WriteFile(filepath.Join(rulesDir, "go.md"),
+		[]byte("# Go Rules\n\ngofmt always"), 0o600))
+
+	ad := New(WithHome(home))
+	res, err := ad.Import(home)
+	require.NoError(t, err)
+	require.Len(t, res.Rules, 1)
+	assert.Equal(t, "go", res.Rules[0].Name)
+	assert.Contains(t, res.Rules[0].Body, "gofmt always")
+}