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/cli/init_test.go

@@ -53,6 +53,34 @@ func TestInitImportWritesCanonical(t *testing.T) {
 	require.NoError(t, err)
 }
 
+func TestInitImportsRules(t *testing.T) {
+	root := t.TempDir()
+	home := t.TempDir()
+
+	reg := adapter.NewRegistry()
+	reg.Register(&importableAdapter{
+		name: "stub",
+		res: &adapter.ImportResult{
+			Rules: []canonical.Rule{
+				{Name: "go", Body: "# Go\n\ngofmt", Path: "go.md"},
+				{Name: "py", Body: "# Py\n\ntypes", Path: "py.md"},
+			},
+		},
+	})
+
+	cmd := NewInit(reg)
+	cmd.SetArgs([]string{"--root", root, "--home", home, "--from", "stub", "--no-prompt"})
+	require.NoError(t, cmd.Execute())
+
+	assert.FileExists(t, filepath.Join(root, "rules", "go.md"))
+	assert.FileExists(t, filepath.Join(root, "rules", "py.md"))
+
+	// Round-trips back through the loader.
+	b, err := canonical.Load(root)
+	require.NoError(t, err)
+	require.Len(t, b.Rules, 2)
+}
+
 func makeStubReg() *adapter.Registry {
 	reg := adapter.NewRegistry()
 	reg.Register(&importableAdapter{