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/cagent.go

@@ -41,6 +41,7 @@ func (a *Adapter) Capabilities() adapter.HarnessCapabilities {
 		ManagesModels:       true,
 		ManagesMCP:          true,
 		ManagesSkills:       false,
+		ManagesRules:        true,
 		ManagesInstructions: true,
 		HasBuiltInSub:       false,
 	}
@@ -57,10 +58,9 @@ func (a *Adapter) Render(b *canonical.Bundle) (*adapter.FileSet, error) {
 	fs := adapter.NewFileSet()
 	cfgPath := filepath.Join(a.Home, ".config", "cagent", "default.yaml")
 
-	instructions := b.Instructions.Global
-	if override, ok := b.Instructions.PerHarness[name]; ok && override != "" {
-		instructions = override
-	}
+	// cagent has no rules concept; the per-agent instruction (system prompt) is
+	// the only sink, so fold rules into it.
+	instructions := b.InstructionTextWithRules(name)
 
 	mcps := map[string]any{}
 	for _, s := range b.MCP.Servers {