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/canonical/instructions.go

@@ -0,0 +1,47 @@
+package canonical
+
+import "strings"
+
+// InstructionText returns the instruction body a harness should receive: the
+// per-harness override when one is set, otherwise the global instructions.
+func (b *Bundle) InstructionText(harness string) string {
+	if override, ok := b.Instructions.PerHarness[harness]; ok && override != "" {
+		return override
+	}
+	return b.Instructions.Global
+}
+
+// InstructionTextWithRules returns InstructionText for the harness with every
+// canonical rule body folded in. Harnesses without a native rules directory
+// (everything except claude-code/kilo) only receive rule content this way —
+// via their global instructions file. Rule frontmatter is stripped so the
+// folded output stays clean; bodies are joined in load order (deterministic).
+func (b *Bundle) InstructionTextWithRules(harness string) string {
+	base := b.InstructionText(harness)
+	var sb strings.Builder
+	sb.WriteString(base)
+	for _, r := range b.Rules {
+		body := strings.TrimSpace(stripFrontmatter(r.Body))
+		if body == "" {
+			continue
+		}
+		if sb.Len() > 0 {
+			sb.WriteString("\n\n")
+		}
+		sb.WriteString(body)
+	}
+	return sb.String()
+}
+
+// stripFrontmatter removes a leading YAML frontmatter block ("---\n…\n---") so
+// folding a rule into a plain instructions file does not leak its metadata.
+func stripFrontmatter(s string) string {
+	if !strings.HasPrefix(s, "---\n") {
+		return s
+	}
+	end := strings.Index(s[4:], "\n---")
+	if end == -1 {
+		return s
+	}
+	return strings.TrimPrefix(s[4+end+len("\n---"):], "\n")
+}