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).

tests/e2e/e2e_test.go

@@ -17,11 +17,15 @@ func TestInitApplyDiffCycle(t *testing.T) {
 	home := t.TempDir()
 	root := filepath.Join(home, ".config", "harness-sync")
 
-	// Fake a claude-code install with one skill
+	// Fake a claude-code install with one skill and one rule
 	require.NoError(t, os.MkdirAll(filepath.Join(home, ".claude", "skills", "hi"), 0o750))
 	require.NoError(t, os.WriteFile(
 		filepath.Join(home, ".claude", "skills", "hi", "SKILL.md"),
 		[]byte("---\nname: hi\n---\nbody"), 0o600))
+	require.NoError(t, os.MkdirAll(filepath.Join(home, ".claude", "rules"), 0o750))
+	require.NoError(t, os.WriteFile(
+		filepath.Join(home, ".claude", "rules", "go.md"),
+		[]byte("# Go Rules\n\ngofmt always"), 0o600))
 
 	run := func(args ...string) ([]byte, error) {
 		cmd := exec.Command(bin, args...)
@@ -33,6 +37,7 @@ func TestInitApplyDiffCycle(t *testing.T) {
 	require.NoError(t, err, string(out))
 	assert.FileExists(t, filepath.Join(root, "config.yaml"))
 	assert.FileExists(t, filepath.Join(root, "skills", "hi", "SKILL.md"))
+	assert.FileExists(t, filepath.Join(root, "rules", "go.md"))
 
 	// apply may produce conflicts on a freshly imported tree (because the
 	// claude-code adapter merges into existing settings.json which differs