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/adapter/adapter.go

@@ -31,6 +31,8 @@ type HarnessCapabilities struct {
 	ManagesModels       bool // writes model selection config
 	ManagesMCP          bool // writes MCP server config
 	ManagesSkills       bool // writes skills symlink or equivalent
+	ManagesAgents       bool // writes agents symlink or equivalent
+	ManagesRules        bool // delivers rules (native rules dir, or folded into instructions)
 	ManagesInstructions bool // writes instructions file (CLAUDE.md, AGENTS.md, etc.)
 	HasBuiltInSub       bool // has its own subscription; provider/model config is skipped
 }
@@ -47,6 +49,7 @@ type ImportResult struct {
 	Instructions string
 	Skills       []canonical.Skill
 	Agents       []canonical.Agent
+	Rules        []canonical.Rule
 	MCP          []canonical.MCPServer
 	Profiles     []canonical.Profile
 }