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/cagent/rules_test.go

@@ -0,0 +1,37 @@
+package cagent
+
+import (
+	"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"
+
+	"gopkg.in/yaml.v3"
+)
+
+func TestCagentFoldsRulesIntoInstruction(t *testing.T) {
+	home := t.TempDir()
+	b := &canonical.Bundle{
+		Instructions: canonical.Instructions{Global: "# global"},
+		Rules:        []canonical.Rule{{Name: "rust", Body: "# Rust\n\nno unwrap"}},
+	}
+	fs, err := New(WithHome(home)).Render(b)
+	require.NoError(t, err)
+	seen := map[string]adapter.File{}
+	fs.ForEach(func(f adapter.File) { seen[f.Dest] = f })
+
+	cfg := seen[filepath.Join(home, ".config", "cagent", "default.yaml")]
+	require.Equal(t, adapter.RenderedFile, cfg.Kind)
+
+	var parsed map[string]any
+	require.NoError(t, yaml.Unmarshal(cfg.Content, &parsed))
+	agents := parsed["agents"].(map[string]any)
+	def := agents["default"].(map[string]any)
+	instr := def["instruction"].(string)
+	assert.Contains(t, instr, "# global")
+	assert.Contains(t, instr, "no unwrap")
+}