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/opencode/opencode.go

@@ -37,6 +37,8 @@ func (a *Adapter) Capabilities() adapter.HarnessCapabilities {
 		ManagesModels:       true,
 		ManagesMCP:          true,
 		ManagesSkills:       true,
+		ManagesAgents:       true,
+		ManagesRules:        true,
 		ManagesInstructions: true,
 		HasBuiltInSub:       false,
 	}
@@ -62,6 +64,12 @@ func (a *Adapter) Render(b *canonical.Bundle) (*adapter.FileSet, error) {
 		Kind:          adapter.SymlinkDir,
 		SymlinkTarget: filepath.Join(b.Root, "skills"),
 	})
+	// opencode reads markdown subagents from ~/.config/opencode/agents/<name>.md
+	fs.Add(adapter.File{
+		Dest:          filepath.Join(base, "agents"),
+		Kind:          adapter.SymlinkDir,
+		SymlinkTarget: filepath.Join(b.Root, "agents"),
+	})
 
 	cfgPath := filepath.Join(base, "opencode.jsonc")
 	existing, err := common.ReadExistingFile(cfgPath)
@@ -104,14 +112,12 @@ func (a *Adapter) Render(b *canonical.Bundle) (*adapter.FileSet, error) {
 		Content: merged,
 	})
 
-	instructions := b.Instructions.Global
-	if override, ok := b.Instructions.PerHarness[name]; ok && override != "" {
-		instructions = override
-	}
+	// opencode has no native rules directory; rule content reaches it only via
+	// the global AGENTS.md instructions file, so fold the rules in here.
 	fs.Add(adapter.File{
 		Dest:    filepath.Join(base, "AGENTS.md"),
 		Kind:    adapter.RenderedFile,
-		Content: []byte(instructions),
+		Content: []byte(b.InstructionTextWithRules(name)),
 	})
 
 	return fs, nil