feat: OpenAPI 3.x backend, one CLI any tool backend

[tomsun28]

Summary

Implements the design in docs/design-openapi-adapter.md: OpenAPI 3.x JSON specs become first-class citizens next to MCP servers, unified behind a neutral Backend contract rather than an openapi→MCP conversion.

agentcli <server> <tool> --flags
          │
    dispatch (backend-agnostic)
          │        ToolDef { name, description, inputSchema }
   ┌──────┴──────┐
 McpBackend    OpenApiBackend
 daemon/direct snapshot → compile → fetch

OpenAPI support

• agentcli server add <name> --openapi <spec.json|url> [--base-url ...] [--header "Authorization: Bearer ${ENV}"]
• Spec compiler: operationId → tool name (mechanical slug fallback, deterministic dedup), path/query/header params + JSON body properties flatten into flags (POST /pets {name,tag} → --name x --tag y), top-level allOf merged, local $ref inlined with cycle decay, external refs rejected with a hint
• Snapshots: spec copied to ~/.agentcli/specs/<name>.json at add time — offline-friendly; --refresh re-pulls from origin (URL or file); corrupt snapshot auto re-pulls
• HTTP → error mapping: 401/403 → AUTH_REQUIRED, 404 → NOT_FOUND, 429/5xx → EXECUTION_ERROR with httpStatus + the API's own error message + retryAfter; ${ENV} header placeholders expand at call time (unset → AUTH_REQUIRED)
• Server help groups operations by spec tag; relative servers URLs resolve against the spec origin
• Zero new dependencies (fetch is built-in; $ref resolver is ~80 lines)

Unification refactor

• Backend interface + neutral ToolResult {data, text?, isError?}; MCP content shapes convert once inside backend/mcp.ts (extractData/parseMaybeEncoded migrated out of dispatch.ts) — the core never sees a protocol-specific shape
• McpTool → ToolDef (adds optional tags, openapiMeta)
• daemon stays MCP-only by design (stateless HTTP gains nothing from it); OpenAPI meta.via is always direct

Docs & positioning

• README: "One CLI. Any tool backend." — MCP + OpenAPI side by side, backend comparison table, OpenAPI section
• SKILL.md: OpenAPI registration + HTTP error mapping notes (still < 6KB, guard test updated)
• package.json: description broadened, version 0.2.0

Test plan

• 41 new tests: 19 compile units (naming/required/$ref/cycles/tags/collisions/relative servers) + 21 live-HTTP e2e (URL building, body flattening, auth env expansion, full error-code matrix, snapshot independence, --refresh, cleanup on remove, Swagger/YAML rejection) + 1 skill guard

• All 48 existing MCP tests pass unchanged (regression: dispatch refactor is behavior-neutral)

• Smoke-tested against the real petstore3.swagger.io spec (19 operations registered; demo API currently 500s server-side for everyone, error mapping verified correct)

• pnpm build clean, pnpm test 89/89