fix(agents): move agents/README.md to docs/agents.md to avoid harness warnings

Tools that scan agents/*.md as custom agent definitions (e.g. GitHub Copilot CLI) parse every Markdown file in agents/ and require YAML frontmatter. agents/README.md was docs, not an agent, so it triggered "malformed custom agent" warnings on every session start.

Moving the docs to docs/agents.md keeps agents/ containing only real agent definitions. Updates internal links in the moved file, in the four persona files, AGENTS.md, and adds a pointer from the root README.

Fixes #258

AGENTS.md

@@ -77,7 +77,7 @@ Composition rule: **the user (or a slash command) is the orchestrator. Personas
 
 The only multi-persona orchestration pattern this repo endorses is **parallel fan-out with a merge step** — used by `/ship` to run `code-reviewer`, `security-auditor`, and `test-engineer` concurrently and synthesize their reports. Do not build a "router" persona that decides which other persona to call; that's the job of slash commands and intent mapping.
 
-See [agents/README.md](agents/README.md) for the decision matrix and [references/orchestration-patterns.md](references/orchestration-patterns.md) for the full pattern catalog.
+See [docs/agents.md](docs/agents.md) for the decision matrix and [references/orchestration-patterns.md](references/orchestration-patterns.md) for the full pattern catalog.
 
 **Claude Code interop:** the personas in `agents/` work as Claude Code subagents (auto-discovered from this plugin's `agents/` directory) and as Agent Teams teammates (referenced by name when spawning). Two platform constraints align with our rules: subagents cannot spawn other subagents, and teams cannot nest. Plugin agents silently ignore the `hooks`, `mcpServers`, and `permissionMode` frontmatter fields.